A really cool little yellow submarine owned by the U.S. Navy and run out of Woods Hole Massachusetts by the Woods Hole Oceanographic Institute.
I'm no Alvin expert, but if I remember correctly, it holds 3 scientists, dives to 6000', and has gone down many hundred times over the past couple of decades. Some of the more colorful dives involved mid-atlantic vents, the Titanic and other shipwrecks, observing jellyfish-like monsters, and rescuing a lost H-Bomb. Alvin is carried to dive sites most often by the Atlantis, a big blue WHOI ship of about 300'.
Every so often Alvin is taken apart in the shop out by the piers in downtown Woods Hole. I haven't seen it happen, but apparently Alvin is stripped completely down to the smallest part, with each part being carefully inspected for safety. Considering the complexity of the machine and the pressures it faces in its extreme operating environment, the thought of a couple of guys in long beards and t-shirts piecing it out on the floor is impressive.