Air-independent propulsion

Air-independent propulsion is a technology that helps submarines stay underwater for much longer than they normally can. Instead of using the air that is above the water's surface to make their engines work, submarines with air independent propulsion use a special fuel called fuel-cells. The fuel-cells combine oxygen and hydrogen to create electricity. This electricity is used to turn a propulsor, which is just like a motor that moves the submarine. With this technology, submarines can stay underneath the water for weeks or even months at a time without having to come up for air.