The Inverted Sea is in the vault of the Darklands layer of Orv known as the Sightless Sea. The vault itself takes its name from the large volume of water which takes up the floor of the vault. However, the Inverted Sea is a much smaller body of water, and is actually on the ceiling (in total defiance of gravity). It is linked to the sea below by the massive "reverse waterspout" known as the Braid.
Above the Inverted Sea is what was once the Azlant Empire, but is now the bottom of the Arcadian Ocean. A massive vent in the Inverted Sea's "bed" (i.e., the ceiling of the vault) links it with the ocean above.
The Inverted Sea is the centre of aboleth activity in the Darklands, and the creatures use powerful magic to ensure the water remains where it is supposed to be. If this magic ever failed, a significant part of the Arcadian Ocean would rush into Orv, with catastrophic results for the ecosystems of both the Darklands and the surface world.
A current flows from the Sightless Sea up the Braid, through the Inverted Sea, into the Arcadian Ocean and then into the Inner Sea. From there it seeps down into Lake Nirthran and eventually finds its way back into the Sightless Sea.