The Lockwood Cellar is located underground in the woods on the property where the old Lockwood Estate was built before it was burnt down many decades ago.
In Bad Moon Rising, Mason sneaked away from the Lockwood Mansion to the cellar, not knowing that Tyler, who was suspicious of his uncle, was following him. After Mason left the cellar, Tyler went down the stairs in order to investigate, where he found an old, dusty room with claw marks and chains mounted onto the walls. Later, Tyler asked his mom about the cellar, but she replied that they don't like to talk about the old cellar because it was used during the time of slavery in the South. When the swimming hole party was being held in the woods near the property, Tyler brought Aimee to the cellar with the intent to hook up with her while Mason was hiding inside. Mason, who had planned to lock himself up in the cellar during the full moon, managed to escape unseen while Aimee explained to Tyler that she couldn't hook up with him because she actually had feelings for Matt.
In Kill or Be Killed, after Sheriff Forbes learned from Mason that Stefan and Damon were vampires, the two worked together to lure the brothers near the cellar so they could be weakened with wooden bullets and vervain. Sheriff Forbes and her deputies dragged Stefan and Damon into the cellar and tortured them in hopes of learning how they walked in sunlight. When it became clear they wouldn't be confessing any information, Sheriff ordered her deputies to kill them by staking them and burning their bodies. However, Caroline and Elena managed to intervene before they could be killed, and Caroline used her vampire powers to kill the deputies when they began to shoot at them, officially outing herself as a vampire to her mother. After Stefan and Damon had begun to heal, the group congregated in the cellar and decided that they would deal with Sheriff Forbes by locking her in the boarding house until the vervain was out of her system, so Damon could compel her to forget what she knew about them.
In The Sacrifice, Tyler and Caroline returned to the cellar to help him prepare for his upcoming first full moon as a werewolf. While the two were looking around, Caroline found a journal that was revealed to be Mason's, which detailed his painful transformations starting from his very first full moon, and which included a USB drive that contained a video of Mason's first transformation after he had turned.
In Ghost World, Mason (in ghost form) and Damon set out to search for something that would help the Mystic Falls Gang defeat Klaus. Their adventures in the tunnels under Mystic Falls led them to find a secret cave within the cellar, which had both Runic and Native drawings on the walls that included the explanation of the Vikings' arrival to the town, the Mikaelson family's history, and the creation of the Original vampires.
In O Come, All Ye Faithful, Niklaus Mikaelson slaughtered twelve of his formerly sired hybrids in the cellar after he was told by Hayley Marshall that they had broken their sire bonds with the intent to overthrow him.