What does a diver use to help them breathe underwater?

Answer 1

The diver uses an air tank, which is a tank filled with compressed air. It has a hose that goes from the tank to the mouth, which is held in the mouth by a 'mouth piece'. The tank has a regulator that allows the pressure in the tank to be regulated down for normal breathing.

Answer 2

Scuba tanks, like a 72 cubic foot, or 80 cf tank, or 100 cf.