Ronon Dex holding the oxygen tank
An oxygen tank is a cylinder which contains pressurized oxygen. There were some of these in the Atlantis infirmary.
In 2008, Ronon Dex and Jennifer Keller were trapped inside the Atlantis infirmary and attempted to use an oxygen tank as an explosive to blow up the door. They were inspired to do this by the movie Jaws, however, contrary to their expectations, the oxygen tank did not explode. (SGA: "Quarantine")