Great question. The pressure in the deep sea can be huge. Here’s the “secret” to how deep sea fish survive. The amount of pressure on the outside isn’t a problem if it equals the pressure on the inside. Let’s do a couple of thought experiments.

1. Imagine that a balloon or bike tire has very little air in it, so the pressure is very low on the inside. If you push on the tire or balloon, you can squash it pretty flat. But if there is a lot of pressure inside, you have to push really hard to even dent it. You can even try this yourself.

2. Now imagine that you filled a container like a bottle with water and dropped it into the ocean (I know you wouldn’t litter, that’s why we’re imagining). If you put a cap on the bottle, as the bottle sank, the pressure outside would become higher than the pressure inside. Eventually, the bottle would be crushed. If you did the same experiment, but left the lid off, the bottle would be safe. As it sank, more water would be forced inside by the pressure. The pressure inside would be as high as the pressure outside, so it wouldn’t be crushed.

Deep sea fish keep the same pressure inside as they do outside. If you collected fish at the bottom of the ocean and brought them up quickly, they’d burst. So when biologists collect deep sea animals, they either have to keep them in high-pressure chambers, or bring them up VERY slowly so that they can equalize their pressure.

Imagine that you took a Styrofoam cup and lowered it to the bottom of the ocean. What do you think would happen? Remember that Styrofoam is made of many tiny CLOSED bubbles. When you have a hypothesis, read this article about the physics of living at the bottom of the ocean:

You may be interested in studying marine biology, physiology, or oceanography.

An important think to realize about deep sea fish is that they cannot survive in shallow water. So the way their bodies work is based on surviving high pressure so they will not survive low pressure. The main way these fish are adapted to high pressure is having different cell membranes. Cell membranes are an oily layer that forms a sort of shell for the cell and in deep sea fish, it’s just made of different oils that are better at dealing with high pressure.

Also, animals that live on the surface have a similar pressure inside their body as the atmosphere, whereas animals in the deep sea have a pressure in their bodies that’s really high. So the problem is the difference between the inside of an animal and the outside. This is why a human can’t survive at high pressures and a deep sea fish can’t survive at low pressures.

Those kind of fish have also gotten rid of gas-filled organs like lungs or swim bladders which would be problematic at high pressures. Other factors that help are generally being small and having a minimal skeleton.

Deep sea fish have chemicals in their bodies that work under that pressure (high). More important, though, is that water (being liquid), does not compress under pressure as air (being gas) does. This means that fish whose bodies are entirely held up by liquid do not have to worry about being crushed, because the water can't be crushed (at least not by any pressure on Earth).