Avalon is a vault in Glastonbury Tor built by Merlin and containing the treasure of all treasures, which was protected by a series of rigorous tests to determine if someone was worthy of receiving it. The vault featured some of the latest Ancient technologies, including beam transporters and solid hologram technology.
Lt. Colonel Cameron Mitchell, Dr. Daniel Jackson, Vala Mal Doran and Teal'c were pursuing this goal when discovering where this location was, thanks to a tablet stolen by Vala. Avalon, it turned out, was actually real and situated in the United Kingdom under Glastonbury Tor, about 125 miles west of London. Under the Tor, Ancient devices protected a cavern where a holographic program in the image of Merlin said that they were to be tested. They endured two tests of their wisdom, while Mitchell fought a holographic guardian as a final test. After they succeeded, a great many treasures were revealed, including a book on Ancient history, a communication device, and arguably the most important item: a datalog detailing Merlin's research on a weapon designed to destroy ascended beings. The only way to access the cavern was through the Transportation rings. (SG1: "Avalon, Part 1", "Avalon, Part 2", "Arthur's Mantle")