You dive deep into the ocean with an oxygen tank strapped to your back. What are you afraid of?

Sharks? Sure.

But you should be even more afraid of water pressure.

Did you know air is pushing on you every day? When people stand on dry land, the air around them has pressure—just the right amount for a person to live in. But water is heavier than air. When divers go deep and breathe from air tanks, they are breathing air with a lot of extra pressure in it. Even the blood and tissue in their bodies are under more pressure than usual.

 If a diver comes to the surface quickly, the pressure releases fast, kind of like bubbles in a can of soda. Those bubbles don’t belong in the human body. They can harm tissue or stop blood vessels from doing their jobs. That makes a person very sick. This diving sickness is called the bends.

Sometimes the bends make someone a little tired or sore in the joints. Sometimes the bends paralyze. Sometimes they kill. One day, a diver might not be affected at all. Another day, the bends could be severe. The deeper a diver goes and the longer he or she stays down, the more gas the diver’s body absorbs.

To dive safely, divers must come to the surface of the water slowly. But coming up slowly can be a challenge when you’re in a hurry to get some air!

Diving marine animals almost never get the bends. God made these creatures with particular abilities to store and redistribute oxygen through their bodies deep underwater. He didn’t design people that way. That’s a good reason for people to come up sloooowwwly—or stay in the shallows. Another way to avoid the bends: Take long breaks between dives.