In the 2003 TV series, this was the original lair that Splinter and the four Turtles lived in for the fifteen years after their mutation. Compared to the later Y'Lyntian outpost and reservoir pumping station, the first lair was a more typical fictional sewer, with a large central space that served as a training hall, living room and bedroom for the four Turtles.
They were forced to leave this lair in the first episode of the series, Things Change, when a small army of Mousers broke through the walls and attacked them. During the battle, the Mousers chewed through the support pillars that kept the ceiling intact, causing a cave-in that separated Splinter from the Turtles. With their home no longer structurally sound, the Turtles and Splinter moved to the Y'Lyntian Outpost, a.k.a. their second lair. In A Better Mousetrap, Leonardo and Raphael were sent back to reclaim what personal possessions survived the cave-in.
However, this ruined first lair eventually gained a new occupant: the mutant crocodile Leatherhead, who was building a transmat device. During a battle with Baxter Stockman, an explosion caused the already-unstable ceiling to cave in completely, permanently destroying what was left of the old lair.