The Tunnels can be accessed through the Lockwood cellar and go under Mystic Falls. The images in the cave are showing the Vikings arrival including the Original Family. The tunnels can also be accessed via Mystic Grill.
Rebekah Mikaelson had been the one to inscribe their names into the wall - some of the later stories were added by other inhabitants of the country, who were witnesses of the Originals and their story. In Ghost World, Mason Lockwood (as a Ghost) accompanies Damon Salvatore deeper into the caverns accessible through the Lockwood Cellar. There they stumble upon a few traps and defenses meant to keep Vampires from entering, most notably the barrier that kept Damon from going near the images left on the walls. The Locked Coffin was also hidden there by Damon, and Alaric later hid his White Oak Stake there as well. The weapon against Originals Mason mentioned turned out to be symbols revealing a second White Oak Tree that would kill them as Rebekah discovered.
In All My Children, Elijah Mikaelson tells Elena Gilbert that there were a series of tunnels underneath Mystic Falls, and all the Originals went underground whenever there was a full moon. Later he tells Elena that he saw his first werewolf there. When Elijah states that Elena is lying and he can hear her heart jump when she lies, he makes a great, big hole in the floor and drags Elena down there. Elijah tells Stefan and Damon that Elena is in the tunnels which led to the cave and they have to stop Esther, he also tells Damon and Stefan that Rebekah is down there with Elena and will kill her if they don't do as Elijah says.
- Elijah's name spelled out on the wall doesn't spell his name, it spells "Mikael".
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