NASA's Titan submarine is a conceptual design for a vehicle that would play a central role in a mission to Saturn's largest moon Titan. This unmanned submersible would be used to explore the oceans of liquid hydrocarbons located at Titan's poles.
Specifically, the Titan submarine would be used to explore the depths of Kraken Mare, the largest polar sea on Titan and one consisting of methane and ethane in a composition akin to liquified natural gas. The sea was discovered by the Cassini probe in 2007, covers an area of 400,000 square km and may be 160 meters deep.
While projects like Titan may seem extravagant and unnecessary to some, the fact of the matter is that humans' curiosity and appetite for exploration is fundamental to their survival, so it only makes sense to dream big and use technological innovation to explore other worlds.