Glimmersea

The Glimmersea, sometimes called the Sea of the Starry Night, was a vast underground ocean lying in the Underdark of Toril.

Geography
The Glimmersea laid some 20 miles (32 kilometers) beneath the floor of another large body of water, the Sea of Fallen Stars in central Faerûn. It stretched out underneath most of the Sea of Fallen Stars, with the exception of the Easting Reach and other parts, and extending beneath some adjoining lands, including Altumbel and Aglarond.

The Glimmersea consisted of a large body of water filling dozens of huge vaults, some thousands of feet (hundreds of meters) high. It was famous for the many ribbon cascades of water that sprayed down from the high ceilings.

Its name, "Sea of the Starry Night", was derived from the luminescent rocks found in the ceilings of some of the vaults of the Glimmersea. Glowing like stars and perfectly reflected in the still waters below, these gave sailors the impression of sailing through a sea of stars, and was a beautiful sight to behold.

Inhabitants
A number of powerful cities of aboleth were situated in the depths of the Glimmersea, and many kuo-toa and sahuagin outposts could be found in the shallows. Needless to say, these feared races kept adventurers at bay and the seemingly tranquil and smooth surface of the Glimmersea was seldom traversed by the ill-prepared.