What's in a scuba tank? Oxygen?

Recreational divers breathe air, not oxygen. The air in a scuba tank is like the air you're breathing but is also filtered to remove impurities.

How long does a tank of air last?

Since people breathe at different rates and as you go deeper you consume air faster, there isn't any one answer. Because of this, divers carry a gauge that lets them know how much air is in the tank. However, divers in calm, warm water from 5-10 metres/15-30 feet can spend about one hour underwater with an average tank.