Before Allen's death in 2018 from non-Hodgkin lymphoma, he was the owner of a fleet of fabulous megayachts. But aboard those luxury yachts was something more exotic: submarines and underwater vehicles capable of diving hundreds of feet into the depths of the ocean.
"It turns out if you go 1,000 feet down in the ocean, it's really dark, and the animals are really strange," Allen told Geekwire in 2011. "But if you put on some Pink Floyd, it's fantastic."
The details of Allen's yachts and submarine were something of a secret, with Fred Rodie, one of Allen's boatbuilders, telling the Seattle Times in 2007 that he's "not really supposed to talk about the sub, but it's a fancy one, a mighty nice piece of work."
Allen's submarine, named Pagoo, was 40 feet long, cost $12 million to build, fit eight passengers and two crew members, and was capable of diving for up to eight hours, according to the Seattle Times and Allen's website. The best part? It was actually yellow.
Allen was passionate about underwater exploration and used his fleet of yachts, subs, and underwater vehicles to help find long-lost artifacts, including the wreckage of multiple World War I and II battleships. He was also present for director James Cameron's 2012 dive to the bottom of the Mariana Trench, according to Geekwire.
And, like many of his peers, Allen had space ambitions of his own. In 2011, he launched Stratolaunch Systems with the goal of building the world's largest airplane in order to launch satellites and send humans to space.