The U.S. Navy submarine, USS F-1, went down after a collision with its sister ship on Dec. 17, 1917, and now sits 1,300 feet below the surface on the ocean floor off San Diego.

The U.S. Navy determined that ascending to the surface without a rebreather was safer, so it started training sailors to free ascend in the '50s with a technique called the buoyant ascent technique.

This allowed the submarine to stay hidden, conserving fuel and reducing its radar signature. The ballast tanks played a crucial role in allowing the submarine to dive and resurface.

By the end of September, Germany's U-9 submarine had sunk a quartet of British cruisers, killing nearly 2,000 sailors. Germany soon announced it would engage in unrestricted submarine warfare.

A submarine's commander can't tell their helmsman to hang a right at the next sea mount to get to their destination. They also can't rise to the surface.

Guests can sleep on the USS Cobia, a World War II submarine, at the Wisconsin Maritime Museum. The submarine is listed on Airbnb and can sleep up to 65 people in sailors' bunks.