(covers information from several alternate timelines)
This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Vulcan's Forge (or simply The Forge) was a vast desert on the planet Vulcan, which played an important role in Vulcan history. The Forge was a natural environment for sehlats. Natural landmarks in The Forge included the Plain of Blood and extensive cave networks.
The environment of The Forge was harsh, subject to electrical sandstorms and possessed geomagnetic instabilities that interfered with 22nd century technology, including scanners, communicators and transporters. The interference extended several hundred meters above the ground and, while aircraft could fly over it, the interference prevented their scanners from identifying objects on the ground.
According to historical record, The Forge was where Surak began his pilgrimage in the 4th century AD and his presumed entrance to The Forge was a location known as "Gateway". In 2154, Jonathan Archer and T'Pol traveled to The Forge to track down members of the Syrrannite movement, thought to be responsible for the bombing of the United Earth Embassy on Vulcan. They discovered that the Syrrannites had taken refuge in the T'Karath Sanctuary, until its destruction by the Vulcan High Command. (ENT: "The Forge", "Awakening")
In 2236, ten-year-old Michael Burnham ran away from her foster parents, Sarek and Amanda Grayson. She was located in The Forge, after her mother appeared to her foster brother, Spock from the future, and showed him Burnham's location. She was rescued by Sarek and the authorities before being slain by a vicious predatory animal. (DIS: "If Memory Serves")
In 2237, a young Spock traveled through Vulcan's Forge during his kahs-wan, or Vulcan maturity test. He visited the location a month prior, to prove to himself that he could endure the ordeal. (TAS: "Yesteryear")
Background information Edit
The name "Vulcan's Forge", based primarily on the Olympian god known to the ancient Greeks (Hellenes) as "Hephaestus" (it was the Romans who called him "Vulcan"), and who was called "The Blacksmith Of The Gods", originated in a Star Trek fanzine from 1969. 
The scenes of Star Trek: Enterprise that are set in The Forge were filmed in the P.W. Gellibrand quarry in Simi Valley, California, on a set built on Stage 9 at Paramount Pictures, and digitally rendered matte paintings. (Star Trek Encyclopedia (4th ed., vol. 1, p. 280); (X)) According to multiple production staffers, The Forge was formed by the nuclear war that devastated Vulcan during the Time of Awakening. To reflect this fact, at one point it was discussed to litter its floor with fragments of rough green glass, similar to those formed at the Trinity nuclear test site in New Mexico. ("The Forge" text commentary, ENT Season 4 DVD & Blu-ray) "Everybody loved it [....] We could have done it for a wide shot, but to actually [have Archer, T'Pol and Syrran] walk across it was gonna be far too expensive," explained Garfield Reeves-Stevens. ("The Forge" audio commentary, ENT Season 4 Blu-ray)