A bathysphere is a spherical tank that allows for undersea exploration. In reality, these devices predated powered submarines and were lowered into the ocean on a cable. The first bathysphere prototype was constructed towards the end of the 1920's, with the first manned dive in 1930. In 1934, the world record for depth of an undersea dive was set in a bathysphere at 3,028 feet, which would remain in place for 15 years. In videogames, the practical possibilites coupled with steampunk aesthetic of the bathysphere make it a good model for undersea travel in exaggerated or alternate versions of reality.
The main mode of transportation in Rapture, bathyspheres are like small underwater trains. Running on tracks, these pods can transport people from one area of the city to another. After the city began to fall into chaos, Andrew Ryan shut down the Rapture Metro, including the bathysphere system. When you arrive in Rapture, you are able to use them because your genetic make-up is close enough to Ryan's to fool the bathysphere security system.
In gameplay terms, the player uses the bathysphere system to progress on their journey through Rapture. The system provides a connection between geographically seperated areas in the gameworld, allowing them to broken up into what would traditionally be viewed as levels. The bathyspheres also allow players to backtrack to previously-cleared areas. The introduction of the game sees Jack finding a bathysphere on the surface; it is in this vessel that the player receives their iconic introduction to the city of Rapture.
When a mutated lungfish kidnaps Lili and drages her into the lake, Raz's inability to enter water prevents traditional pursuit. He uses a bathysphere to descend into Lake Oblongata and discover the lair of the lungfish.
In the levels "Down The Tubes" and "Tube Race" Earthworm Jim must safely navigate an extremely fragile bathysphere through narrow, underwater caverns.