Complex sentence: Momsen returned to his diving bell idea in 1930, built a prototype, constructed from a water-tight aircraft hangar pirated from submarine "S-1" and tested it off Key West, Florida.
Split into simpler sentences: Momsen returned to his diving bell idea in 1930. He built a prototype, constructed from a water-tight aircraft hangar pirated from "S-1" and tested it off Key West, Florida.