Alright, who here has a healthy heaping of thalassophobia? If you don’t know anything about the deep sea, you’re about to get a healthy heaping of it. You may know that we’ve only explored like 10% of the ocean, which means there’s an absolute mess of stuff we have yet to know about our own planet. You might also know that if you go deep enough, no sunlight or food makes it down deep enough into the sea. Which would indicate that life is hard there, and if things are alive, they’d be small. But some things are big. Like, 33-feet-long and 1,500 pounds of squid big. With so little food and light, how do deep sea creatures get so big? And how deep is the ocean, anyway?
And if you’re curious, also check out: How Deep is the World’s Deepest Hole?
The Challenger Deep
Have you ever looked down when you’re taking a swim, only to notice you can’t see the bottom of the pool, lake, or part of ocean you’re in? If that doesn’t fill you with an immense sense of anxiety and dread, then you’re stronger than most of us. At least in a pool they tell you how deep the thing is.
True, we do at least kind of know how deep the ocean can get. But unfortunately, you’re probably not going to like how deep it gets.
For perspective, the Titanic was found about 2.4 miles beneath the surface (about 3.8 km). That’s pretty deep, right? Like by that point it’s already pitch black.
Except the deepest part of the ocean we’ve made it to is the Challenger Deep, a point near the southern end of the Mariana Trench in the Pacific Ocean. It’s so deep less people have been down there than we’ve sent to the Moon. How deep is “less people than the Moon” you might ask? 6.8 miles (11 km) deep. That’s deeper than Mount Everest is tall (5.5 miles, 8.9 km)!
Zoning the Deep Ocean
What the layperson thinks of as the ocean generally falls under the epipelagic zone. It only goes down about 650 feet (0.2 km). Which is like 2% of the Challenger Deep. Colloquially, it’s referred to as the “sunlight zone,” since this is almost the limit of the Sun’s reach.
Once you’ve gone past the sunlight, you’ve entered the mesopelagic zone. This will take you from the bottom of the sunlight zone to around 3,300 feet (1 km) deep. By this point, sunlight is pretty faint, so you’re going to start seeing those fun and funky glowing fish. We fittingly call this the “twilight zone”. You haven’t technically entered another dimension, but it might as well be one.
Past the twilight zone comes the aptly named “midnight zone,” also known as the bathypelagic zone. You’ll make it down to 13,000 feet (4 km), and you’re not going to see any sunlight. The only light you’ll be seeing is from bioluminescent creatures, or your own flashlights. Because it’s dark, most of the life you’ll see is black or red–since there’s no reason to be colorful if nobody can see you. Oh, if you’re wondering why red is a possible color of choice, that’s because red is of the longest wavelength in the visible spectrum, so it dissipates the easiest. It’s basically black for the purposes of the fish.
We Have to go Deeper
Existential crisis time brings us the abyssopelagic zone (the “abyssal zone”), taking us down to about 19,500 feet (6 km). Most of the life here is invertebrate thanks to the pressure, and the water temperature is basically freezing.
But it goes deeper, to the hadalpelagic zone. Here we’ve just got… The bottoms of the trenches. They take us to the deepest parts of our ocean trenches, like the Challenger Deep. By this point, the water pressure is 8 tons per square inch. That’s literally like being crushed by 48 Boeing 747 jets from all directions at once. Surprisingly, and perhaps miraculously, we’ve found life even down here.
Honestly, quick side note, if life is going to persist even here, there’s no way Earth is the only planet with life on it, right?
Unfortunately, we’ve found a little more than just life at the bottom of the Challenger Deep. We’ve also found plastic bags. So we guess pollution knows no bounds.
Dang that’s depressing.
We can’t tease you with giant sea monsters in our intro and not talk about them a little. Scientists have found that some deep sea creatures can get big. Like stupid big. We mentioned that 33-foot-long, 1,500 pound beast known as the colossal squid. Because squids weren’t already scary enough. And yes, the colossal squid is bigger than the oft-mentioned giant squid. So basically, the Kraken is real.
Anyway, deep sea creatures exist at really high pressures (see that Boeing 747 thing we mentioned earlier in this post). Because of this pressure, the big creatures normally don’t have skeletons because, well, they would be crushed.
We also know food is super scarce this far down in the ocean, so you’d think that a lot of organisms this far down would be small. Smaller bodies means they need less food to sustain themselves, right?
Wrong. Well, actually, right–but only sort of. We can thank Kleiber’s Law, which essentially means bigger animals have more efficient metabolisms. Basically big things need to eat a lot less. It’s observable too, giant isopods in captivity can go like 5 years in captivity without food. 5 years!
There’s also food storage. Deep sea creatures, like the giant isopod, eat a lot when they get the chance. We’re talking so much food they literally can’t move anymore.
We should also consider the relative lack of predators. Since there are so few other animals deep down in the abyss, nobody’s gonna be around to eat you while you’re getting big. Then you get really big, and nobody is physically large enough to eat you. Thus, sea monsters.
Need more deep sea giants? Here’s a quiz about them.