Hydrogen and helium are two elements that are kinda-sorta interchangeable in airships.
Hydrogen was once used to fill airships. Quite dangerous due to how flammable it is.
Helium
Hydrogen was the gas that was once used to filled airships. It is extremely plentiful and easy to produce. However, it is also extremely flammable and caused a few accidents. Helium is now used.
The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed that Hydrogen gas was too dangerous(it is extremely flammable) to be used. Since then, Helium has been used instead. Helium is also a very light element, and is completely safe, as it is inert.
Hydrogen (explosive), Helium (non-explosive).
Hydrogen used to be used in airships.
gas is still used in airships to provide lift.
That'd be hydrogen.
Airships are also, and more commonly, known as zeppelins. The term airship is also used to refer to an airplane, but this usage is incorrect, as they have two different meanings. Airships might also be called blimps or dirigibles.
Hydrogen
Hayao Miyazaki has featured airships in several of his films, including Nausicaa of the Valley of the Wind and Castle in the sky. You can also find airships in Howl's moving castle. Here is a website on some anime with airships: http://animeworld.com/related/airships.html Also, here is a website with a lot of other movies with airships: http://spot.colorado.edu/~dziadeck/airship/films.htm
because hydrogen used to be in those airships and since hydrogen is flammable it was very dangerous