The sea has always interested people. From it they can get food, minerals, and treasures. For thousands of years, they sailed on the sea. But they could not go far beneath its surface. People want to explore deep into the sea. However, they are not fishes. Because people must breathe air, they cannot stay under the water's surface for any length of time. To explore deep water, men and women face even more dangers and problems. A diver who wants to stay under water for more than a few minutes must breathe air or a special mixture of gases. Divers can wear diving suits and have air pumped to them from above. They can carry tanks of air on their back and breathe through a hose and a mouthpiece. Water weighs 800 times as much as air. Tons of water push against a diver deep in the sea. The diver's body is under great pressure. When divers are under great pressure, their blood takes in some of the gases they breathe. As they rise too quickly, the gases in their blood form bubbles. Divers then suffer from the bends. The bends can cause divers to double up in pain. Bends can even kill divers.