We know more about the surface of Mars than about the deepest reaches of Earth’s oceans.

But even though the lower depths of the oceans have gone largely unexplored, we still have some understanding about the evolution of deep-sea fish. Despite the darkness, immense water pressure and frigid temperatures of the deep sea, a panoply of fish, squid, sharks and other aquatic life display characteristics that explain how and why they exist in what seems like an unforgiving place.

From anglerfish that use bacteria to create a glowing lure to capture prey, to fish that have a “superpower” vision that allows them to see that glow, deep-sea life is fascinating and full of specimens that are worth illuminating.

The Deep Is Pressure-Packed and Dark

To understand the living environment of these unique creatures, it might help to review the uniqueness of ocean depths. As the nonprofit MarineBio Conservation Society explained, the area of the sea where floating and swimming organisms live — the open-water realm called “pelagic” — runs between 200 to 11,000 meters deep. Sunlight dwindles as you go down and disappears at around 1,000 meters of depth. The other realm, known as “benthic,” is the bottom of a sea. Here, there are three levels of deepness, and the lowest of lows in an ocean is the hadopelagic, where deep trenches are 11,000 meters, or 36,000 feet, deep. This place of seemingly no return is known as the hadal.

Despite complete darkness in the hadal, pockets of light exist, generated by the organisms there, a chemical reaction called bioluminescence. But if the dark isn’t all-enveloping, the water pressure certainly is. Every 10 meters in depth is one unit of atmosphere (atm), which is the equivalent of 14.696 psi. So, at the bottom, where pressure is more than 1,100 atm, that means all life feels the crush of 16,166 psi. That’s like flipping the Eiffel Tower upside down and placing the weight of the tip on someone’s toe.

You would think at this level, the hadal proper, life would be unbearable — but it’s not. Here organisms swim just above the floor, burrow into or are attached to it. It’s home sweet home to a wide array of life. How these creatures live and why they’re attracted to this end of the ocean keeps scientists busy, but inevitably frustrated. Even though technological advancements are allowing explorers to go deeper than ever before, it’s still not deep enough to see life firsthand. Time will tell if equipment can eventually withstand the Eiffel Tower-heavy pressures of the deep, deep sea.

Until then, scientists will continue studying organisms from the hadal through circuitous methods, like looking at the genes of related fish and making educated connections or, when luck will have it, seeing the creatures themselves when they wash ashore.

Super-Sized and Super-Sighted Fish

Even with limited access, scientists are able to see how the depth, darkness and frigid temperatures of the hadal has shaped the evolution of deep-sea fish.

For instance, the recent spotting in the Gulf of Mexico of a 10-foot-long squid, one that’s bigger than most others, revived scientific talk about a landmark 2006 study that posits many creatures down in the deep sea are relatively larger because of water pressure. Maybe they’ve been flattened. The study also forwards the idea that the colder the temperature, the more of a natural increase in size. Also, as the theory goes, big-bodied creatures got big to handle the long spells of time in which there’s little food to eat at such depths.

If being big wasn’t bold enough, how about deep-sea fish with “superpower” vision? Fish at deep-sea level have a genetic disposition for having more rod ospins, which are retinal proteins that detect dim light. That dim glow comes off fish, bacteria, shrimp and other life that carry a faint bioluminescence. Most vertebrae couldn’t detect this light but fish living the deep life evolved to have this so-called superpower. Meanwhile, scientists determined the anglerfish uses its special glow to lure other fish for quick snacks. That prey could probably benefit from a superspeed to get away.

As remarkable as these discoveries are, it’s just scratching the surface of deep-sea life. Further studies and improvements in deep-sea technology hopefully will uncover even more fascinating finds.