The Y'lyntian outpost
The Turtle Lair is the main home of the Ninja Turtles. This was the second home that was featured in the show, the first being a more ordinary sewer where they spent the first decade and a half of their lives. When this lair was rendered unsafe and unstable by the Mousers, Splinter proposed the move to the new lair, which he had found while he was escaping from the Mousers. Things Change
This new lair was much larger and more comfortable than their old one, with a bedroom for each of the brothers, an area for Donatello's lab, a large living area with an entertainment system, and an outlet into the ocean. It also had a mysterious, hidden elevator that led to the upper world (below an abandoned garage/repair shop), which none of the Turtles initially could explain. This abandoned garage is located on the corner of Eastman and Laird, a reference to the original creators of the Turtles. Meet Casey Jones
The structure was eventually revealed to be of Y'Lyntian origin, perhaps as some kind of outpost of the underground city. The Entity Below
The Turtles' Y'Lyntian outpost lair was destroyed by Karai and the Foot Clan in an act to avenge the Utrom Shredder's exile. Scion of the Shredder. Upon Leonardo's return to New York City, he pulled his family together and took them to the Reservoir Station, making it their new lair. Prodigal Son
They later return to the outpost to meet with their enemies and allies to battle the Tengu Shredder. Enter the Dragons, Part 1