The Cracks of Doom or Crack of Doom were a great fissure, a deep chasm filled with fire, in which the One Ring was forged by Sauron and was destroyed when Gollum fell with the One Ring into it.
Geography
The Cracks of Doom were located in the Sammath Naur, the Chambers of Fire, a long cave with a high roof high up on the cone of Mount Doom on the plain of Gorgoroth in the land of Mordor.
From a dark entrance in the eastern side of Mount Doom, the door of the Sammath Naur, a long cave led inside until the great fissure of the Cracks of Doom cut across the floor and walls of the cave a short way from the door. A road led from the west gate of Barad-dûr, the Dark Tower of Sauron, to the door of the Sammath Naur.
Sauron forged the One Ring in the Cracks of Doom in c. S.A. 1600, and put so much of his own force into the Ring that the only way it could be destroyed would be to throw it back into the Cracks of Doom. Such an opportunity came in S.A. 3441 after the Battle of Dagorlad but Isildur kept the Ring for himself, against the counsel of Elrond and Cirdan.
It was decided in the Council of Elrond that the One Ring must be brought to the Cracks of Doom to be destroyed. Frodo Baggins and Samwise Gamgee reached the Cracks of Doom on 25 March, T.A. 3019. It was there that Frodo decided to keep the Ring and put it on his finger and where Gollum bit off the finger with the Ring and fell with it into the fiery chasm destroying the Ring.