A free ascent is when a scuba diver with compressed gas in his system leaves his diving depth for the surface without his breathing apparatus. The recommended rate of ascent is 18 metres (60 ft) per minute. As the gases in the diver's lungs expand, they need to be exhaled rapidly enough to prevent over-inflation of the lungs. Free ascent is a technique that requires a great deal of training; military divers are sometimes trained in this technique. Some diver training organizations also include training for a free ascent (controlled emergency swimming ascent) in their syllabuses. Free ascent is an emergency procedure that involves risk but could save a diver's life from an otherwise hopeless situation.