“Welcome to the Underbelly, brothers. Home of the Flushed Pets!” — Snowball, The Secret Life of Pets
The Underbelly is the home base of the Flushed Pets.
Description[]
The sewer lair is located under Manhattan, New York City. It is a wide open space in the sewers of New York City, with many former pets living in it. As the sewer extends in all directions, the animals can move anywhere just like the road system in the human world; they are seen climbing up to the road (for saving Ripper) and going to River Hudson (when they pursue Max and Duke).
The lair is of all sorts of thrown or neglected materials (such as pipes, chains, and wood), by which the animals built for the residence for themselves. There is a square with wood floor that is for Snowball, the leader, to announce news to all members of The Flushed Pets, and the place is piled up higher and surrounded by wastewater. In addition, a corner of the wall acts as a memorial of a late member, Ricky, with his photos and white candles placed around; a large pipe is exclusively for Viper, who seems to be inside in normal times.
Though Snowball becomes a pet again because the adoption by Molly, it can be assumed that the Underbelly is still in use, as seen that Tattoo, Derick, and Dragon flee to the sewer again, even if the leader is absent. As for The Flushed Pets, they are later seen as guest to the party in Leonard's Apartment.