The Lightless Reaches is an underwater trench and acts as a border between the Shimmering Expanse and Kelp'thar Forest. The water currents through here are said to be very strong. Seahorses can be found swimming through here.
As it wraps around within the Shimmering Expanse it serves to separate Biel'aran Ridge from the main portion of the underwater zone.
During the World of Warcraft: Cataclysm beta, this area was known as The Abyss.