No one would believe it if a diver had not caught this

Perhaps the first thing a scuba diver thinks of when dealing with pressure is tank pressure. Scuba tanks contain a lot of air in a relatively small volume, and the only way to do this is to compress the air, producing high pressure. A diver can determine the amount of air left in a tank by using a pressure gauge. Usually, a full tank has a pressure of 3,000 pounds per square inch (psi). If you get below 200 psi, you should be out of the water.

Normal air—the stuff that blankets the Earth—is mostly nitrogen molecules, which make up about 79 percent of it. The rest is oxygen, at around 21 percent. We can imagine that these molecules are like super-tiny balls moving at different speeds and in different directions. If this gas was in a container, some of the molecules would collide with the wall, bounce off of it, and change direction. This change in motion means that each molecule exerts a small force on the wall. (A bigger wall or container will experience more collisions and a greater overall force.)

If you measure the force in pounds and the area in square inches, you get pressure in pounds per square inch, or psi. That's the most common unit for tank pressure in the United States.

Another unit is the bar, where 1 bar is equal to 14.5 psi. The value of 1 bar is very close to the pressure of air on Earth. The atmospheric pressure of the air that surrounds you right now is probably 14.5 psi. (Yes, I said "probably" because I don't want to judge you. Maybe you are reading this from the top of Mount Everest, where the pressure is just 4.9 psi, because there is less air above you pushing down. If so , send me a picture.) In terms of force and area, it is equal to 100,000 newtons per square meter.

Water is also made of tiny moving molecules that act like balls, and those molecules collide with underwater objects (like people), producing pressure. Water has many more molecules than the same volume of air, which means there are more collisions to produce greater pressure. But just like going to the top of Mount Everest decreases the air pressure, going deeper in water increases the pressure, because gravity pulls downward on the molecules of water. For every 10 meters of depth, the pressure increases by 1 bar, or 14.5 psi. That means that on a dive 20 meters (around 60 feet) below sea level, there would be a water pressure of 43.5 psi, three times greater than the air pressure at Earth's surface.

(The fact that pressure increases with depth prevents all the ocean's water from collapsing into an infinitely thin layer. Because the pressure is greater the deeper you go, the water underneath pushes up more than the water above it pushes down. This difference compensates for the downward gravitational force, so the water level stays constant.)

It might sound like 43.5 psi is too much for a person to handle, but it's actually not that bad. Human bodies are very adaptable to changes in pressure. If you have been to the bottom of a swimming pool, you already know the answer to this pressure problem—your ears. If the water pressure on the outside of your eardrum is greater than the pressure from the air inside your inner ear, the membrane will stretch and it can really hurt. But there is a nice trick to fix this: If you push air into your middle ear cavity by pinching your nose closed while attempting to blow air out of it, air will be forced into this cavity. With more air in the inner ear, the pressure on both sides of the membrane will be equal and you will feel normal. This is called "equalization," for hopefully obvious reasons.

There's actually another air space that you need to equalize while diving—the inside of your scuba mask. Don't forget to add air to it as you go deeper, or that thing will awkwardly squish your face.

There is one other physics mistake a diver could make. It's possible to create an enclosed air space in your lungs by holding your breath. Suppose you hold your breath at a depth of 20 meters and then move up to a depth of 10 meters. The pressure inside your lungs will remain the same during this ascent, because you have the same lung volume, and they contain the same amount of air. However, the water pressure outside of them will decrease. The reduced external pressure on your lungs makes it as though they are overinflated. This can cause tears in lung tissue, or even force air into the bloodstream, which is officially bad stuff.