A planet-sized sphere of water in the Delta Quadrant, kept intact by an ancient field generator at its centre and settled by the Moneans, which features in the Voyager episode Thirty Days.
Monea, which its inhabitants call the Waters, is a huge globe of ocean with no solid core. In the episode Thirty Days, set in 2375, Voyager meets the Moneans, a formerly nomadic people who settled it about three centuries earlier and built underwater cities.
The ocean is slowly losing volume, and Tom Paris, a lifelong lover of the sea, takes the Delta Flyer down to its centre with the Monean scientist Riga. They find an ancient reactor of unknown origin generating the containment field, and learn that the Moneans' own oxygen refineries are weakening it.
When the Monean authorities, represented by the consul Burkus, decline to act, Paris and Riga try to destroy a refinery. Janeway stops them with a torpedo, reduces Paris to ensign and sentences him to thirty days in the brig.