The Depths of Eternity was the bottom of the Lake of Apparitions, a lake found beyond shadows, a realm of the Force which Force-sensitives entered after separating their minds from their physical beings. When walking across the lake beings had to stay on a set path, or they would fall into the Depths of Eternity, a fall from which they could not be saved or retrieved.
Jedi Grand Master Luke Skywalker and son, Jedi Knight Ben Skywalker, traveled to Sinkhole Station in search of the Mind Walkers in 43.5 ABY. The Mind Walkers took them beyond shadows, where eventually they found the Lake of Apparitions. While crossing the lake, Luke stumbled, causing his whole leg to go underwater. When Ben looked to see the cause, he saw his father had stumbled at the sight of Mara Jade Skywalker floating on the surface.
Later, both Skywalkers Mind Walked there again, along with Sith High Lord Sarasu Taalon, Sith Saber Gavar Khai and Captain Leeha Faal. Faal was dragged down into the Depths by long-dead enemies, which caused her physical body to spasm violently and then die.