The Fuel Can is a common pickup found in caves (in all depths). Once collected, it will fill the Submarine's Fuel Tank by a small amount, allowing the driver to use a speed boost by pressing spacebar which will consume the fuel in the tank. The Maximum quantity of fuel allowed to be stored in the Fuel Tank can be increased with the Fuel Tank Upgrade, which will also fully refill the Tank upon pickup. Any extra fuel collected while the Tank is full will be lost.
Update News Overview
Fuel! - Introducing a new resource into the Deeper world - Fuel! The formerly electric vessels are now hybrids, with Fuel allowing you to cruise at your higher Engine speeds and powering your powerful EMP shots. Spend fuel wisely, though - if you run out you'll be operating on pure electric power, capping your max get-away speed, and EMP's will no longer be operable.
Have no fear however, for you will be able to refill your tank and even upgrade its capacity with Fuel Canisters and Upgrades you can find in caves!
Update Link (includes image): http://steamcommunity.com/gid/103582791455126886/announcements/detail/1438188152909674283