When a submarine implodes, the fittings and pipes give way as the intense water pressure crushes the hull, similar to the process of a fist crushing an egg or a lemon. All of the people inside die in a matter of seconds.
One American submarine that actually imploded was the U.S.S. Thresher, a nuclear attack submarine. The craft was undergoing diving trials off the shore of Cape Cod, Massachusetts, under the watchful eye of the sub rescue ship U.S.S. Skylark when the Thresher came to a swift end.
The Thresher radioed to the Skylark that it was undergoing some minor problems. After that, the Skylark received some more messages, but they were incomplete and garbled. Then, there was a brief silence, and after that, the Skylark heard the sounds of a submarine breaking up and imploding. From the first sign of a problem to the implosion took just five minutes.
The U.S. military conducted an investigation and concluded that a piping joint in the engine room's sea water system failed, spraying water that shorted out the electronic systems and causing the nuclear reactor to shut down. Without any power, the sub likely sank below its crush depth, and the pressure of the sea caused the implosion.