The Cracks of Doom is the name given to the volcanic fissure in Mount Doom, where Frodo Baggins had to throw the One Ring (Ring of Power) into to be destroyed. It was the only place in Middle earth where the One Ring could be destroyed. These facts are specifically noted in the novel, The Lord of the Rings by J. R. R. Tolkien.
Also, it is held in The Lord of the Rings that one of the most powerful artifacts ever created in Middle earth, the One Ring was crafted by the Dark Lord Sauron during the Second Age. His intent was to enhance his own power, and to exercise control over the other Rings of Power, which had been made by Celebrimbor and his people with Sauron's assistance. In this way, he hoped to gain lordship over the Elves and all of the other races in Middle earth.