Gollum's lake was the lake in Gollum's cave where he lived for many decades.
The lake was formed by water that had seeped through the rock forming a cave. The residue of the sedimentary rock washed down to the bottom. It was wide, deep, and piercingly cold, yet fish somehow lived in it. The lake also has an outlet to the outside via an underground stream.
Gollum lived off of the lake and the fish that lived in it. He paddled a homemade boat back and forth between the lake and a small island at the center of the lake, which was also Gollum's home. The Goblin King would occasionally send orcs down to get fish, and Gollum would kill the orc. The orcs, however, did not seem to suspect Gollum's existence.
It first appears in The Hobbit in the chapter Riddles In The Dark, as the meeting place of Gollum and Bilbo, after the latter discovers the former's lost "property:" the One Ring. There, the two engage in a battle of riddles, which ends when Bilbo asks Gollum what is in his own pocket.
|Lakes of Arda|
Bywater pool | Forbidden Pool | Gollum's lake | Great Lake | Kheled-zâram (Mirrormere) | Lake Evendim | Lake Helevorn | Lake Mithrim | Linaewen | Long Lake | Lórellin | Nen Hithoel | Nísinen | Tarn Aeluin