Six hundred metres below the surface, the ocean is not quite dark. And that is the problem.

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[00:00:00]This fish has a secret. It is hiding in plain sight. To this barracuda, it is invisible. This is the hatchet fish.

[00:00:10]Predators pass within inches. They see nothing. In the twilight zone, faint light filters down from above. To anything hunting from below, you are a shadow against the light. But, the hatchet fish has a trick, counterillumination.

[00:00:27]Tiny light organs on its belly produce a glow exactly matching the light filtering down from the surface. No shadow. No silhouette. Nothing to find. It can sense the light from the surface and adjust its own glow to match it precisely.

[00:00:46]The result is perfect camouflage in open water with nothing to hide against in the most dangerous place on Earth.

[00:00:57]It is perfectly safe.