Some TMNT stuff was always meant for grownups.
In the Mirage TMNT continuity, the first sewer lair is the lair where Hamato Splinter raised his four adopted mutant turtle sons Leonardo, Donatello, Raphael and Michelangelo from first mutation and earliest childhood. Later, Leatherhead moves in after Splinter and the turtles move out, and from then on it becomes Leatherhead's lair.
This lair features in various TMNT stories that take place during the turtles' earlier lives (such as Not Forgotten), but it must be abandoned after volume 1 issue 2 when it is discovered and partially destroyed by Baxter Stockman's Mousers. The turtles are given a new home in April O'Neil's apartment in the Second Time Around building, and Splinter joins them later after the destruction of TCRI.
More than two years later, after having lived over a year in April's apartment and then more than another year at the Jones Farm, in Return to New York, Book One, Raphael returns to New York City on his own, and in Zog, Raph allies himself with Zog, a Triceraton straggler from the destruction of TCRI who is going insane in Earth's less breathable atmosphere. Raph and Zog live temporarily in the ruins of the old lair as they prepare to attack the Foot Clan. Raph's brothers rejoin him and they successfully kill Oroku Saki's clone (with Zog getting killed in the process), but the family opts not to move back into their wrecked compromised first lair, and instead finds a second sewer lair to live in.
Later, during the events of Leatherhead, the turtles befriend Leatherhead, a mutant alligator ally of the Utroms who had been left behind when TCRI was evacuated. Leatherhead fashions a sewer lair of his own, but it is discovered and compromised by cryptid hunter Jack Marlin, forcing Leatherhead to move. Leatherhead's new turtle friends give him the remains of their first sewer lair for him to live with, and it becomes Leatherhead's lair.
In this lair, Leatherhead gradually tries to construct a transmat device in the hopes of eventually rejoining his adoptive Utrom family on their homeworld.
- This lair's appearance often differs from artist to artist.