The Crevasse of Memories That Will Be, (PROSE: Cat's Cradle: Time's Crucible) later known as the Sepulchasm (PROSE: Lungbarrow) and also in some way synonymous with the Ginnunga Gap of Norse mythology, was an unfathomably deep opening in the skin of Gallifrey which was located in the Cavern of Prophecy beneath the Pythia's Temple. For the entire reign of the Pythias, a wicker cage was suspended over the Crevasse for a Pythia to sit in and absorb the vapours from the Crevasse which aided in clairvoyance and clairaudiance. This process was aided by a concealed pipe which would spray water vapour at the lip of the Crevasse, creating clouds which would surround the cage. (PROSE: Cat's Cradle: Time's Crucible) Somewhere deep in the pit there were thought to live constrictors. (PROSE: Lungbarrow)
The mental dimension which the Pythias would access from the Crevasse contained the Gate of the Past or Future. From this vantage point, as once witnessed by Ace, the Crevasse resembled a bottomless crack in the universe in which Time poured dust of time itself, slowly filling the void of the Old Time universe with chronology like an hourglass.
The 508th Pythia spent considerable time over the Crevasse, desperately trying to see the future which was stolen from her by Rassilon. After spending several weeks locked in the cage, the Pythia lost all hope after touching the mind of the Seventh Doctor and cast Pythia's Curse. She then cut the rope suspending the cage and plunged into the Crevasse. (PROSE: Cat's Cradle: Time's Crucible)
The Pythia's Temple would be forgotten, with the Capitol built over it. (PROSE: Lungbarrow) By the Doctor's time, history of the Crevasse had shifted enough that it was said that Rassilon had cast the Pythia into the pit, (PROSE: Cat's Cradle: Time's Crucible) with such a history performed annually at the Mystery of the New Time ceremony. This ceremony depicted the Crevasse as the Sepulchasm, a prominent catastrophe-associated concept in Gallifreyan culture due to a board game of the same name which depicted the Sepulchasm opening up and swallowing the Gallifreyan dead. The Mystery of the New Time depicted that following the mass death of infants after the Pythia's Curse many Gallifreyans threw their stillborn children into the Crevasse. (PROSE: Lungbarrow)
When the Decayed Master opened the Eye of Harmony, it caused a disruption in the earth below the Capitol, briefly opening a "chasm" in the Panopticon. The Master fell down this chasm, screaming. (PROSE: Doctor Who and the Deadly Assassin)