Airships are popular forms of transport in steampunk stories. The hot air balloon is the oldest successful human-carrying flight technology. It is part of a class of aircraft known as balloon aircraft. Hot air balloons that can be propelled through the air rather than just being pushed along by the wind are known as airships or, more specifically, thermal airships. An airship or dirigible is a lighter-than-air aircraft that can be directed on its course through the air using rudders and propellers or other thrust. Unlike other aerodynamic aircraft, such as fixed-wing aircraft and helicopters, airships and balloons stay aloft by filling a large cavity, such as a balloon, with a lifting gas. Rigid airships are the most common in steampunk stories. These airships are a great deal larger than their softer counterparts, and can cover greater distances, giving them the ability to cross most of Earth's oceans.