潜水艇是军事上一种重要的舰艇．它可以潜入水下航行用于侦察敌情、攻击敌舰．和平时期还可用于海底考察，水下运输等建设目的．潜艇的沉浮是靠改变自身重来实现的．潜艇据要上浮时，就用压缩空气排出水舱中的水使潜艇变轻，从而浮出水面．但若潜艇沉在海底或大江河底的泥沙之 A submarine is an important military vessel. It can dive underwater to scout the enemy and attack enemy ships. In peacetime, it can also be used for underwater exploration, underwater transportation and other construction purposes. The ups and downs of submarines are achieved by changing their own weight. When the submarine is going up, it uses compressed air to discharge the water in the tank to make the submarine lighter and float to the surface. However, if submarines sink to the bottom of the sea or sediments