Submarines where used in the military ever since world war 1. From that time they began to get used a lot more comonly and evolve at the same time. The submarines that where made by the military usually got named after countries, cities or war heroes. The military's submarine weapon usually is the torpedo(a short missile with TNT inside of it), but some have heavy machine guns and tomhawk missles on top of them, some can even carry nuclear warheads. But now how will these navy's best machines last in a life after people?
1 day after people: At the abandone naval yards the submarines sit quietly, meanwhile in the open waters some submarines still swim but without a crew those subs will soon crash into the coral reefs below. And at the naval history muesums the submarines also remain suspesiosly queit.
2 days after people: The submarines that where in the open sea, are starting to get dangerously closer to the coral reefs and soon the crash. Creating holes inside that let the water in and soon they sink to the bottom. Elsewhere the submarines that where tyed up on the docks and the ones inside museums appear to be okay. But for how long?
20 years after people: The weakend ropes can not hold the subs anymore, soon they brake off and float away. Meanwhile at the meuseums the submarines start to deteriorate.
50 years after people: The submarines that broke off from their docks are starting to get holes on the sides, thus making some of them sink.