When a submarine implodes, the fittings and pipes give way when the hull is crushed by the strong water pressure, much like a fist crushing an egg or a lemon. Everyone within perishes in a couple of seconds.
The nuclear attack submarine USS Thresher was one of the American submarines that really collapsed. When the Thresher went down, it was undertaking diving trials off the coast of Cape Cod, Massachusetts, under the watchful eye of the sub rescue ship U.S.S. Skylark.
The Thresher radioed the Skylark to let them know it was having some minor issues. The Skylark received more signals after that, but they were partial and jumbled. The Skylark then heard the sounds of a submarine breaking up and collapsing, followed by a momentary stillness. It took only five minutes from the first sign of a problem until the implosion.
A piping joint in the engine room’s sea water system burst, shooting water that shorted out the electronic systems and caused the nuclear reactor to shut down, according to an examination by the US Navy. The sub most likely sank below its crush depth without any power, and the pressure of the sea caused the implosion.