Divers... are a specialization that has absolutely nothing to do with submarines. Our sub had a diver who was either a scuba diver or a hard hat diver. He was a Machinist Mate who had attended a specific scuba diving training school (Naval Diving and Salvage Training Center Home Page). Submarine diving is different from surface ship diving in several important ways. Submarines are underwater for most of their time, so they need special equipment for atmospheric pressure instead of the high-pressure air tanks used by aircraft and surface ships. They also use complex systems of valves, tunnels, and passages instead of lungs and ears to control how much oxygen they give off. Submarines are also hidden from view, which makes them difficult targets for enemy weapons. A submarine can only be seen by another submarine or vessel using sonar signals, and even then only if the other boat flies its flag proudly or not at all. Finally, submarines must operate alone or in small groups called "teams". This is necessary because a single torpedo or bomb could destroy an entire submarine crew without leaving a trace of damage on the surrounding environment. The same thing happens when a submarine goes undetected while hiding in plain sight. The American football term "sub" refers to a player who lies flat on his back without moving who is therefore not eligible to receive further action in the game.