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The Sightless Sea is by far the largest of the known vaults of Orv. It lies beneath the Arcadian Ocean, and is of a similar size. There are numerous tiny islands in the sea, many settled by urdefhan but others claimed by even stranger creatures. On average, the sea is a mile deep, but in places it is much deeper.
The aboleth are the most powerful faction in the Sightless Sea. At one time they ruled it all, but today they are apparently content with just controlling most of the depths. Other foul creatures live in the deep regions outside of aboleth territory; the shoggoth are perhaps the worst, and may even pre-date the aboleth.
A current from the Sightless Sea flows 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. In theory this grants the aboleth access to most of Golarion, although there have never been any confirmed sightings of aboleth coming down with the rain that lands in the Midnight Mountains.