The Barrow Deeps was an ancient cave system beneath Mount Hyjal in Warcraft III. It was the sleeping place of the Druids of the Claw for thousands of years until the return of the Burning Legion and the Third War. After awakening the Druids of the Talon from their slumber, Malfurion Stormrage and Tyrande Whisperwind entered the Barrow Deeps in search of the remaining Druids.
Deep in the caves, the night elves discovered giant spiders which were mutated by corruption in the roots of Hyjal, and Tyrande saved a Furbolg Shaman after he had been bitten by the creature. They also came across the dwelling of The Largest Panda Ever, which would attack if disturbed.
Having nearly forgotten after all the years, Malfurion and Tyrande found the gates to the prison of Illidan Stormrage, Malfurion's brother. Despite adamant protests from Malfurion, Tyrande entered the prison. She believed that Illidan would be a powerful ally against the Legion so she split off from Malfurion and went in search of his imprisoned brother.
Malfurion continued on in search of the Druids of the Claw. He soon found them but they were lost and feral, their minds gone into atrophy. The Druids had slept for so long in their bear forms that they remained as such and even began to believe that they were bears. Malfurion freed them from their hibernation with the Horn of Cenarius. Grateful to be awake and in their right minds once more, the Druids of the Claw agreed to follow Malfurion in the fight against the demonic Legion.
Meanwhile, Tyrande had freed Illidan from his prison. When reunited with his brother, Malfurion refused to trust Illidan after his betrayal long ago but accepted the aid offered.
This section concerns content exclusive to Cataclysm.
From quests in Mount Hyjal it is revealed that Fandral Staghelm has been placed in prison. Due to the power he possesses, members of the Twilight's Hammer clan are attempting to capture him. Players are tasked with the responsibility of removing him from the Hyjal Barrow Dens so that he might be transported to Moonglade. This ends up being a ruse by the , who has been corrupted by the Twilight's Hammer clan. She assists Fandral in his escape, and he becomes one of Ragnaros' followers.
It is possible that the Barrow Deeps are only a small part of a vast cave network extending across most of Northern Kalimdor, connecting Felwood, Azshara, Moonglade, Winterspring, the Mount Hyjal and even with Ashenvale. In fact, the uncorrupted furbolgs of the Timbermaw Hold have a side tunnel, currently blocked by a door, which leads to an unknown location. These tunnels supposedly harbor a small portion of the furbolg race that have managed to escape corruption. In Warcraft III the Barrow Deeps were occupied by at least one tribe of uncorrupted furbolgs fueling speculation these tunnels and the Deeps may be one and the same.