The Abyss was created due to earthquakes, and was a yawning, impassible gorge between Loamhedge Abbey and the rest of the Southern Plateau. It is unknown how deep it is, but is assumed to go to the bottom of the earth's crust. At one point in time, some creatures managed to construct a bridge that was hung low into the abyss. During the time of Mattimeo, Slagar and the woodlanders crossed the abyss to get to Malkariss, Slagar then burned the bridge so that Matthias and his companions could not follow him. During this time, the last bridge was lost, leaving it impassible for all those without wings. With the help of Sir Harry the Muse, the woodlanders recovered the remains of the bridge, then used their belts as a zip line to get across. The woodlanders' return was not specifically mentioned but likely involved the makeshift zip line constructed with the remains of the bridge.
When Malkariss' kingdom collapsed, its inhabitants were thought to have been crushed in the cataclysmic event. In Loamhedge however, it appeared that some of the deeper tunnels remained and were ruled by descendants of the Wearet. These tunnels opened out into the gorge where the Lord of Mossflower had fallen and was used as a new bridge. This was called 'The Abyss' by the Wearet's followers. These vermin were vanquished in the Battle in the Abyss.