As Lake Tahoe’s Water Levels Dropped, Divers Discovered Something Terrifying

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[00:00:00]You can now take a hike in the water of beautiful Lake Tahoe. California State Parks have developed Tahoe's first underwater trail. It's called the Emerald Bay State >> There is a lake on the border of California and Nevada, so clear you can see 60 ft down [music] from the surface on a calm day. For generations, people have looked into that water and seen nothing but beauty, but the water has been dropping. And as it drops, things that were never meant to be seen are being exposed, [music] ancient things, preserved things, things that have been sitting in the darkness of that lake floor for thousands of years waiting.

[00:00:32]What divers found when they went down to look is not what anyone expected. Hi, my name is Matthew [music] and this is Reef Discovery. Lake Tahoe, what everyone thinks they know. [music] Here is what most people think they know about Lake Tahoe, because the postcard version of this place is genuinely impressive enough that it is easy to never look beyond it. Lake Tahoe sits at 6,225 [music] ft above sea level in the Sierra Nevada mountain range, straddling the border between California and Nevada. [music] It is the largest alpine lake in North America. It holds enough water that if you spread it evenly across the entire state of California, the state would be under 14 in of water. The lake is approximately 22 mi long, 12 mi wide, [music] and in places over 1,600 ft deep. That depth point, by the way, makes Lake Tahoe one of the deepest lakes in the United States. Only Crater Lake in Oregon goes deeper, but the thing that makes Tahoe genuinely famous, the thing that has drawn tourists and scientists and writers and artists for over a century is the clarity of the water. On a calm, clear day, you can see objects 60 ft below the surface from a boat. [music] In the 1960s and '70s, before development and runoff began affecting water quality, visibility measurements exceeded 100 ft in some areas. The water is so clear, so impossibly blue, that first-time visitors often stop and stare because their brains struggle to process what they are [music] seeing. It does not look like water. It looks like blue glass. Mark Twain visited Lake Tahoe in 1861 [music] and described it as the fairest picture the whole earth affords.

[00:02:07]John Muir called it the brightest of all mountain lakes. Today, the lake draws over 15 million visitors [music] every year. There are luxury resorts on its shores, celebrity homes worth tens of millions of dollars, and ski [music] resorts in the mountains above it. It is by any measure one of the most beautiful places in [music] North America. That is the version of Lake Tahoe that most people carry in their heads. Beautiful, clear, permanent, [music] a jewel of the American West that has always been there and always will be. That version of the story leaves out a great deal. The drought that changed everything. To understand what divers started finding, you need to understand [music] what has been happening to Lake Tahoe's water level over the past two decades. And I want to be precise about this because the numbers [music] matter. Lake Tahoe has a legal minimum water level. It is not just a scientific measurement. It is a legally defined threshold established by water rights agreements between California and Nevada. That threshold sits at 6,223.0 ft above sea level. When the lake drops below that line, water [music] stops flowing out through the Truckee River, which serves downstream communities and ecosystems. Below that line, the lake [music] is essentially in crisis mode.

[00:03:17]In the summer of 2021, Lake Tahoe dropped below that legal threshold for the first time in over 20 years. By the fall of 2021, the lake was sitting at its lowest level since [music] 2009. The following year brought modest recovery, but the broader trend line over the preceding two decades told a disturbing story.

[00:03:37]California and Nevada had been locked in [music] a persistent, punishing drought cycle driven by reduced Sierra Nevada snowpack, hotter summers that accelerated evaporation, and changing precipitation patterns that scientists were already linking to [music] long-term climate shifts. What does it actually look like when a lake loses that much water? If you had visited Tahoe regularly over the years, you would notice it in small accumulating ways. Beaches [music] that used to disappear under the summer waterline were now exposed year-round. Boat ramps that had always reached the water were ending on dry ground. Rocky outcroppings that old photographs showed surrounded by water are now standing in open air.

[00:04:15]The marina docks had been extended, [music] then extended again, and the shoreline was receding. Not dramatically, not apocalyptically, [music] but steadily, relentlessly, in a way that felt permanent rather than seasonal. For casual visitors, this was mildly concerning, something to shake your head at and mention at dinner. For the scientific diving community working in Lake Tahoe, the dropping water levels meant something [music] entirely different. They meant visibility. They meant access. They meant that sections of the lake floor that had always been at the edge of safe recreational diving depth were now reachable. Water that had always been at the extreme boundary of what divers could safely explore was becoming accessible. So, they went [music] down to look. The divers go deeper. The diving community at Lake Tahoe is not large, but it is dedicated.

[00:05:02]The lake presents unique challenges that keep casual divers away. The altitude means reduced oxygen availability at the surface. The water is cold year-round, [music] hovering around 40° Fahrenheit at depth regardless of the season. And the very clarity that makes the lake beautiful also creates a psychological hazard called the Martini effect. The unusual clarity combined with altitude and cold can cause nitrogen narcosis to set >> [music] >> in faster than divers expect, creating a dangerous disorientation that has contributed to accidents over the years.

[00:05:34][music] But, for experienced cold water divers and scientific teams, these challenges were manageable, [music] and the dropping water levels of the early 2020s created a specific window of opportunity. Areas of the lake floor that had previously required technical deep diving were now within reach of advanced recreational divers. [music] Scientific teams began conducting systematic surveys of sections of the lakebed they had never been able to document properly before. [music] What they expected to find was interesting but unremarkable geological features. Perhaps some debris from the lake's history of human activity, old equipment, remnants of the [music] steamship era when boats crisscrossed the lake in the late 1800s. Maybe some artifacts from the resort development of the 20th century. [music] The kind of historically interesting but scientifically routine material that accumulates on the floors of lakes [music] near human settlements. What they found instead stopped them cold, and I mean that in every sense of the word. Standing on the lake floor at depths that had been largely inaccessible [music] until the water dropped were trees. Not fallen trees, not timber, not debris. Standing trees, upright, rooted in the sediment, [music] ancient beyond any reasonable expectation. Still intact. One diver who participated in early surveys of these formations later described the experience in terms [music] that I find impossible to improve on. She said, "Going down there and seeing those trees standing in the darkness felt less like diving and more like walking into a room that someone had just left. Everything was still there. Everything was exactly where it had been left. It was just that the last time anyone had been in that room was several thousand years ago." I have been researching underwater discoveries for years now. And that description [music] gave me chills the first time I read it. Still does. The forest under the water. Here [snorts] is what the divers were actually looking at and why it matters so much. Submerged forests are not unheard of in geological terms. When water levels rise, whether through flooding, glacial [music] melt, or tectonic shifts, forests can be inundated and preserved underwater. It happens.

[00:07:36]>> [music] >> There are submerged forests documented off the coast of Wales, in in across Scandinavia, in reservoirs created by modern dams. This is a known phenomenon.

[00:07:45]What made the Lake Tahoe discovery different was the condition of the trees and what that [music] condition implied about when and how they got there. These were not waterlogged, decomposed stumps.

[00:07:56]>> [music] >> They were not the kind of ghostly remnants you might expect after centuries of submersion. The cold temperature of the deep lake, consistently near freezing at depth, combined with the low oxygen environment [music] and the unusual chemical composition of Tahoe's water that slowed biological decomposition >> [music] >> to an almost complete standstill. The trees were preserved in a state that researchers described as extraordinary.

[00:08:20]Bark still intact [music] on some specimens, branch structures still recognizable, root systems still embedded in the sediment exactly as they had been when the trees were alive.

[00:08:28][music] And they were standing upright. This is the crucial detail. A fallen, [music] transported log can end up anywhere. It tells you relatively little about what happened to it. But a tree standing upright, still rooted [music] in its original position, tells you something very specific. It tells you that the forest did not flood suddenly. The trees were alive and growing in that location.

[00:08:49]The water rose around them. They died in place, and then they stayed there, standing in the darkness and the cold for thousands of years. The dating of these trees is where [music] the story becomes genuinely disturbing. Early analysis of samples from the submerged trees placed their age at somewhere between [music] 6,000 and 10,000 years old. Let me give you the context to understand what that means. 6,000 years ago, the Egyptian Old Kingdom did not yet exist. [music] The wheel had only recently been invented in Mesopotamia. Stonehenge had not [music] been built. The entire span of recorded human history fits inside the age of some of these trees with room to spare. [music] 10,000 years ago puts us at the very end of the last ice age.

[00:09:31]The Pleistocene epic was drawing to a close. [music] Megafauna like woolly mammoths and ground sloths were going extinct across North America. The climate was shifting dramatically [music] and rapidly. And in what is now the bottom of Lake Tahoe, a forest was growing. Healthy trees and soil [music] that is now under hundreds of feet of water. What happened between then and now is the question that drives everything else in this story. [music] And the answer, as best as scientists can determine it, is both straightforward and terrifying. The water rose. Not gradually over millennia [music] in a way that would be easy to dismiss. The evidence suggests the rise happened in episodes. Periods of catastrophic flooding and climate disruption that inundated [music] the forest faster than the trees could respond, trapping them in the sediment at the bottom of a lake that would grow deeper and colder and darker over the centuries that followed. Those trees are not just interesting. They are evidence of a world that was >> [music] >> erased. A landscape that looked completely different from what exists today. And they have been sitting down there in the dark this whole time, [music] perfectly preserved, waiting for the water to drop far enough that someone could finally come and find them. The bodies. I need to talk about something that most Lake Tahoe coverage politely avoids. [music] And I apologize in advance because this section is genuinely unsettling. Lake Tahoe has a reputation among divers, [music] among local historians, and among the people who have lived around the lake for generations. It is a reputation that rarely makes it into the tourist brochures. The reputation [music] is this. Lake Tahoe does not give up its dead. This is not folklore. It is a documented physical phenomenon [music] with a straight forward scientific explanation that does nothing to make it less disturbing. In most lakes, when a body enters the water and sinks [music] to the bottom, decomposition processes generate gas inside the body over time.

[00:11:17]That gas eventually causes the body to become buoyant again >> [music] >> and rise to the surface, where it is typically found within days or weeks.

[00:11:24]This is why drowning victims are usually recovered relatively quickly in most fresh water lakes. Lake Tahoe's extreme depth and near freezing temperatures at depth prevent this process from occurring. The cold water slows decomposition so dramatically that gas production never reaches the level needed for the body to rise. Instead, [music] bodies that sink to depth in Lake Tahoe remain at the bottom, preserved by the cold and conditions not entirely unlike refrigeration, essentially [music] indefinitely. There are accounts stretching back to the early days of European settlement around the lake of divers and fishermen reporting encounters with preserved bodies at depth. [music] Some accounts are vague and unverifiable. Others come from professional divers with documented dive records who reported their discoveries to authorities. In 1973, a diver conducting a survey near the south end of the lake reported encountering what he described [music] as multiple figures on the lake floor at significant depth. His account was [music] investigated and then, as these things tend to go, quietly filed away.

[00:12:28]The dropping water levels of the recent drought period brought some areas of the deeper lake floor into closer proximity to recreational [music] diving depth than they had previously been. And with that increased accessibility came renewed accounts from divers of unexpected encounters [music] at depth. I want to be careful here because I am not going to sensationalize individual cases or repeat unverified claims. What I will say is this, the scientific reality of cold water preservation at depth in Lake Tahoe [music] is not disputed. The lake's history of drowning accidents, combined with the documented physical phenomenon of non-surfacing in cold deep water, means that the lake floor almost certainly contains remains from accidents stretching back decades. The dropping water levels made parts of that floor more accessible than they had ever been during the modern diving era. What divers found when they went to those newly accessible depths is something that authorities and researchers have [music] been understandably reluctant to discuss in detail. I I leave it at that.

[00:13:26]Some discoveries are better left where they are found. What the trees [music] actually tell us, let us come back to the trees because the scientific story they tell [music] goes much deeper than simply confirming that the lake used to be smaller. Scientists [music] who have studied the submerged forest at Lake Tahoe have used a technique called dendrochronology, the analysis of tree rings on samples from the preserved specimens. [music] If you are not familiar with dendrochronology, here is the short version. Trees add one growth ring per year. Wide rings [music] indicate good growing years with plentiful water and warmth. Narrow rings indicate drought years or cold years. By reading the sequence of rings in a tree's cross-section, scientists can reconstruct the detailed record of the climate conditions the tree experienced during its lifetime. The tree rings from the Lake Tahoe submerged forest told a story that researchers described as striking even by the standards of paleoclimate research, which is a field that deals with striking findings on a regular basis. The trees showed evidence of multiple severe drought periods in the centuries [music] before they died.

[00:14:29]Extended stretches of narrow rings indicating years, [music] sometimes decades, of dramatically reduced precipitation. And then, at the end of many of the samples, a sudden shift. The final rings before the trees were inundated showed a transition [music] from drought conditions to something else entirely. Conditions suggesting sudden, massive water input. A climate event of significant scale that brought the drought to an abrupt end, not through gradual recovery, but through a kind of violent correction. [music] The evidence suggested that the Sierra Nevada region had experienced what paleoclimatologists call [music] a mega drought followed by a dramatic wet period in the period roughly 7,000 to 9,000 years ago. The drought phase was severe enough and long enough to lower Lake Tahoe's levels dramatically, allowing forests to establish themselves on what is now the lake floor. The subsequent wet phase was [music] abrupt and powerful enough to raise those levels rapidly, drowning the forest >> [music] >> before the trees could retreat. Here is the part that should make you sit down whatever you are eating. The drought conditions at the submerged trees recorded in their rings bear a significant resemblance to the conditions [music] that modern climate scientists are projecting for California and Nevada over the coming century under various climate [music] change scenarios. The pattern of extended drought followed by sudden extreme precipitation events is not a prehistoric anomaly. [music] It is a documented recurring feature of the Sierra Nevada climate system. Lake Tahoe's submerged forest is [music] not just a record of the past. It is a possible preview of the future. The water dropped once before, dramatically, long enough for an entire forest [music] to grow on the lake floor. Then it came back. The question that no one wants to answer out loud is whether we are watching the beginning [music] of the next cycle. The hidden structures. Now, we get to the part of this story that the scientific community has been most reluctant to discuss openly. And I want to be very clear about what I am about to say and what I am not saying [music] because this is the section where it would be easy to veer into territory that I have no interest in. I am not going to tell you that aliens [music] built something at the bottom of Lake Tahoe. I am not going to use the word ancient in a way that implies anything other than its literal meaning. What I am going to do is tell you what divers and researchers have reported, what the evidence shows, and where the [music] genuine scientific uncertainty lies. In the course of conducting surveys of the newly accessible sections of the lake floor, [music] multiple dive teams reported encountering formations that did not match the expected geological profile [music] of the area. Lake Tahoe sits in a graben, a geologically technical term for a valley [music] formed by fault blocks dropping relative to the surrounding terrain. The lake [music] was formed by tectonic activity and has a geological history that scientists understand fairly well. The expected lake floor features are sediment layers, exposed rock faces consistent with the fault geology, debris cones from the surrounding mountains, [music] and the kind of gradual topographic variation you would predict from a tectonically formed alpine lake. What some dive teams reported finding [music] instead were formations with a geometric regularity that did not fit neatly into the expected geological picture.

[00:17:37]Arrangements of large stones and patterns that several researchers independently described as unusually linear. Flat surfaces at angles and orientations that struck investigators as inconsistent with natural rock deposition. These formations were documented photographically and with sonar mapping equipment. Now, before anyone gets carried away, and I say this as someone [music] who spent an embarrassing amount of time going down research rabbit holes on this particular [music] aspect of the story, there are completely natural processes that can create geometrically regular rock formations. Columnar jointing, for example, where cooling lava fractures into hexagonal or rectangular [music] columns is a well-documented phenomenon.

[00:18:17]Frost wedging and other physical weathering processes can create surprisingly regular patterns in [music] rock. The geological history of the Lake Tahoe Basin includes volcanic activity that could produce natural formations with unusual geometry. [music] The honest answer is that not enough systematic investigation has been conducted [music] on these formations to say anything definitive about their origin. The preliminary sonar data is intriguing. The photographic documentation raises legitimate questions. The researchers who have seen the formations in person use careful, qualified language when describing them, which in scientific communication is often a signal that someone [music] has seen something they cannot yet explain and is being appropriately cautious rather than dismissive. [music] What is not in dispute is this. The dropping water levels revealed sections of the Lake Tahoe floor that had never been properly mapped or documented. What is on that floor in those sections is not yet fully understood. And the combination of the ancient submerged forest, the geological complexity of the [music] basin, and the possibility that humans lived in and around this region during the periods when the lake floor was dry [music] land means that the question of what lies at the bottom of Lake Tahoe is considerably more open than it was [music] 20 years ago. There is a site about 2 mi offshore from the western shore that shows up in sonar data as a roughly rectangular elevated feature approximately 40 m across. It sits at a depth that would have been dry land during the period when the submerged trees were [music] alive and growing. Researchers have flagged it for further investigation. That investigation has not yet been fully [music] completed and published. I am going to leave that detail sitting right there and let you do with it what you will. What it all means. Let me try to pull all of this together because individually [music] each piece of this story is striking, but collectively they add up to something that I think deserves [music] to be taken very seriously. Lake Tahoe is not just a beautiful alpine lake. It is a record, a physical archive of climate events going [music] back 10,000 years written in submerged wood and sediment and preserved in cold dark water. The dropping water levels of the recent drought [music] period did not create these discoveries. They revealed them.

[00:20:26]Everything that divers found had been down there the entire time, invisible and inaccessible, [music] waiting. The submerged forest tells us that the world we think of as permanent and stable [music] is neither. The Sierra Nevada landscape that exists today, the lake, the forests, the mountain snowpack, the river systems, is [music] one configuration of a region that has looked radically different in the past and will look radically different again in the future. The trees standing on that lake floor grew in a world that is gone. [music] The climate that killed them eventually produced the world we live in and the climate patterns we are currently creating are beginning to resemble in disturbing ways the [music] patterns that created the conditions under which those trees grew and died.

[00:21:09]The preservation phenomenon that makes Lake Tahoe such a challenging environment for recovery operations is a reminder that this lake has its own rules. It is extraordinarily deep, extraordinarily [music] cold, and in many ways extraordinarily unforgiving.

[00:21:24]The clarity that makes it beautiful from the surface is deceptive. Below a certain depth, Lake Tahoe is a place of darkness and cold that operates entirely outside human experience of it. What is down there? What has accumulated over decades and centuries of human activity around the lake is not fully known. The geometric formations on the lake floor represent either a fascinating natural geological phenomenon, a remnant of human activity during [music] a period when that section of the lake floor was dry land, or something in between that will require serious scientific investigation to properly characterize.

[00:22:00]None of those possibilities is uninteresting. All of them deserve attention. [music] And here is the thing that stays with me most from all of this research. The [music] lake is still dropping. The drought conditions that drove the water level down to historic lows have eased somewhat in recent years, but the long-term trend driven by warming temperatures, reduced snowpack, and shifting precipitation patterns [music] has not reversed. The sections of the lake floor that became newly accessible to divers over the past decade may be only the beginning. If the water continues to drop, and climate projections suggest it [music] will, over time scales of decades rather than centuries, then sections of the lake bed that are currently far beyond safe diving depths will eventually come within [music] reach. We may be in the early stages of a long process of revelation. Lake Tahoe has been hiding things for a [music] very long time. The water dropping is forcing those things into the light. What we discover next will depend on how far the water falls and how seriously we take the responsibility of investigating what is down there before the conditions that are preserving these discoveries [music] begin to change. Here is what Jeremy I mean here is what I keep coming back to.

[00:23:10]I actually had a whole different ending [music] written for this video and then I scrapped it because the more I sat with this story the more I felt like the real ending was not [music] about the discoveries themselves. It was about what the discoveries represent. Lake Tahoe is one of the most studied, most photographed, most visited bodies of water [music] in North America. It has been here in front of us the entire time and we had [snorts] no idea what was at the bottom of it. We had no idea that a 10,000 year old forest was standing in the dark below the tour boats and the paddle [music] boards and the celebrity waterfront properties. We had no idea what the sediment layers in its depths were recording about the climate history of this continent. We did not know because we did not look. [music] We looked at the surface and we thought we understood the whole thing. The water dropping forced us to look deeper and what we found there was not reassuring.

[00:23:58]I think about those trees sometimes.

[00:24:00]Standing in the dark, rooted in sediment, preserved [music] in the cold for 10,000 years. They were alive during a world that no longer exists. The climate events that killed them and the [music] lake rising around them erased that world completely. And then the lake started dropping again and we went down [music] and found them. That is either a warning or it is a coincidence. I know which one I think it is. If you found this video useful, share it with someone who needs to hear this story. And if you [music] were new here, welcome to Reef Discovery. We go to the places most people do not think to look. The water is always hiding something. I am Matthew. I will see you in the next one.