Gato Class Ballast System: Negative Tank by Mike Keating

Negative Tank - I'll quote directly from the manual:

"Negative tank, located inside MBT 2A and 2B, is used to provide negative buoyancy for a quick dive."

"Negative tank is built to withstand full submergence pressure and can be used as a variable tank..." It's connected to the trim system.

"The snorkel induction mast is drained to the negative tank (when) preparing to snorkel." The same amount of water is blown out of negative at the finish of snorkeling, when the snorkel induction mast refills, with water.