Liquid hydrogen
Hydrogen in any state is very bulky; it is typically stored as a deeply cryogenic liquid, a technique mastered in the early 1950s as part of the hydrogen bomb development program at Los Alamos. Liquid hydrogen is stored and transported without boil-off, because helium, which has a lower boiling point than hydrogen, acts as cooling refrigerant. Only when hydrogen is loaded on a launch vehicle, where no refrigeration exists, it vents to the atmosphere.