Scuba diving is where a diver goes underwater using a breathing apparatus which independent of a air support system separate from air above water.
I have never been scuba diving before! I have always wanted to go.
Scuba divers sometimes use additional tanks or cylinders for decompresion gas or to use when an emergency occurs.
That's good they shpuld always have a back up plan!
There are closed circut or semi closed rebreather systems which allow people to breath recycling of breathed out gases.
I know we breathe out carbon dioxide which is toxic so thats good that they don't breathe it back in.
The volume of gas used compared to open circuit versus smaller cylinder may be used for an equivalent on dive duration.
I know a lot of gas can fit in a small volume when under pressure.
open circuit scuba systems usually discharge the breathing of gas into the surrounding environment and its exhaled and consists of multiple diving cylinders which provides the gas breathed at a higher pressure through a regulator.