Cracks of Doom
From the One Wiki to Rule Them All, the Lord of the Rings Encyclopedia.
The Cracks of Doom is the name for the volcanic fissure in Mount Doom, where Frodo Baggins had to throw the One Ring into to be destroyed. It was the only place in Middle earth where the One Ring could be destroyed.
It is described in The Return of the King (novel) as a great fissure that split a long tunnel that bored into Mount Doom. From the Crack of Doom, a red light, presumably from lava, is emitted. The red glow is the only light in the cavern, as even the Phial of Galadriel is dimmed by the power of Sauron in the heart of his realm. There is a road leading from Barad-dûr to the Crack of Doom.
Frodo claims the One Ring here; he and Gollum fight over it. Gollum falls into the Crack of Doom with the ring, thus completing the Fellowship of the Ring's quest.
In The Fellowship of the Ring (film), Elrond leads Isildur to the Crack of Doom to destroy the One Ring immediately after it's cut from Sauron's hand but Isildur refuses.
