Since the lungs are filled with gases, they are compressed under pressure. At 100 meters, the lungs are compressed to the size of oranges.
The greatest pressure change is near the surface. At 30 meters, the pressure is 4X that at the surface, including 15 psi of pressure from the atmosphere.
At () at sea, there're pressing on each square inch of your body. It's like a person standing on your chest.
The lungs would be of normal size if it were not for the blood shift to the core.
When divers started regularly diving past 50 meters, it became clear that something was preventing fatal collapse of the lungs as predicted by the formula.
As the lungs are compressed, blood moves to the chest. Since blood is not compressible, it keeps the chest cavity from crushing completely. The spleen also contracts, releasing more blood.
In contrast, SCUBA fills a diver's lungs with compressed air, which acts against the ambient pressure. The downside of this (besides the heavy equipement) is that compressed air contains more nitrogen and oxygen, which creates the three big risks of deep SCUBA diving: Nitrogen Narcosis, Decompression Sickeness, and Oxygen Toxicity.
Method 1: If you're able, you can try exhaling all the air in your lungs. Then while holding your breath, do a few more forced exhalations. It will feel like someone is standing on your chest.
Method 2 (only with supervision!): Freedivers also practice "negatives" in the pool to further increase the effects of pressure. A diver exhales completely and sinks to the bottom. In a shallow, pool you can lie right on the bottom. Since the lungs are empty, the pressure at even 1 meter creates an intense sense of pressure. The vacuum in the chest pulls the air from the throat cavity.
Many competitive deep divers suffer an injury called "squeeze", usually minor. When lungs are extremely compressed, lung damage may occur. Most of the time, divers come up, spit out a bit of blood, and are fine. It can manifest as a slight cough. Diving without adequate training or ignoring warning signs can lead to squeeze. If you suspect you've experienced sqeeze, abort diving and rest. Fatalities from squeeze are rare even in extreme freedivers and are preventable.
The sinuses are another air-filled space that compresses and expands with pressure (think of your ears popping on an air plane). To prevent the ear drums from rupturing, divers use the tongue to draw air from the lungs and push the air into the inner ear.
At great depths equalization becomes increasingly difficult. Not only is there less air volume in the lungs to draw out, the vacuum in the lungs and throat makes it hard to even move the tongue. Ear injuries are not uncommon. One controversial technique is to flood the inner ear with water instead of air, eliminating the air space.
Photo credit: Gabriel Barathieu