Hydrogen is the earth’s lightest element, and it can be obtained easily and inexpensively, but its flammability makes it unacceptable for manned airship operations.Dozens of hydrogen airships were destroyed by fire, and no American airship has been inflated with hydrogen since the crash of the U.S. Army airship Roma in 1922. The use of hydrogen as a lifting gas for passenger airships was completely abandoned by the late 1930s.