The whale is a mammal(哺乳动物) warm-blooded, air-breathing, giving birth to its young alive, sucking them—and, like all mammals, originated on land. There are many signs of this. (51) front flippers (鳍肢), used for steering and stability, are traces of feet. Immense strength is built into the great body of the big whales, and in fact most of a whale's body is one gigantic muscle. The blue whale's pulling (52) has been estimated at 400 horsepower. One specimen was reported to have (53) a whaling vessel for seven hours at the (54) of eight knots. An enraged whale will attack a ship. A famous (55) of this was the fate of whaler Essex, which was sunk (56) the coast of South America early in the last century. More recently, steel ships have had their plates buckled (扭曲变形) in the same way. Sperm whales were known to (57) the old-time whaleboats in their jaws and crush them. The greatest mystery of whales is their diving ability. The sperm whale can (58) the bottom for his favorite food, the octopus (章鱼). (59) that search he is known to go as far down as 3,200 feet, where the pressure is 1,400 pounds, to the square inch. Doing (60) he will remain submerged (水下的) as long as one hour. Two feats (绝技) are involved in this: storing up enough (61) (all whales are air-breathed) and withstanding the great change in pressure. Just (62) he does it scientists have not determined. It is believed that some of the oxygen is stored in a special system of blood vessels, rather than just held in the lungs. And (63) is believed that a special kind of oil in his head is some sort of compensating mechanism that automatically adjusts the internal pressure of his body. But (64) you can't bring a live whale into the laboratory for study, no one (65) just how these things work.