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Strap on your scuba gear, we're going for a dive! We're going all the way to the deepest depths of the oceans. Well, maybe not. If you have an ample supply of oxygen, why can't you dive to the ocean floor? The answer lies in one word: pressure.
Pressure is the amount of force applied to a given area. At sea level, the air exerts about 14.7 pounds per square inch (or psi) of pressure on everything it touches. That means that each square inch of your body has about 14.7 pounds of force pressing on it.
But once you don your scuba gear and jump into the sea, things change. The pressure in the water is greater because water is denser than air and exerts more force. For every 33 feet you go down into the water, the pressure increases another 14.7 psi, or one more atmosphere (atm). With the proper training and enough oxygen in your tank, you can dive down three or four atmospheres, but beyond that you will need a submarine to do your exploring. Why is that? Humans and most other mammals have lungs that hold only a small amount of air to push back against the water's pressure. As you swim farther down into the water, the increasing pressure causes greater force against your body. At some point, the outside, inward pressure overcomes the outward pressure exerted by the lungs, and the rib cage collapses. This would kill a human being!
Then how does sea life survive in the deep? Whales often dive to depths of 7,000 feet—that's more than 200 atmospheres—to hunt for squid. They can do this because their ribs are more flexible since they are made of cartilage and because their lungs can collapse safely under pressure. The whale's unique adaptations help it maintain a balance between inward and outward pressure. As for other deep-sea animals, pressure has little effect on them, because they don't have lungs. In addition, their flesh is soft and jelly-like, making them more buoyant and resistant to the underwater pressure.
So the next time you watch a scuba diver, you'll know how far that diver can go—and why the animal kingdom wins at a game of "Underwater Survivor."
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