The Cistern is the penultimate dwelling place of Pennywise the Clown and the location where the Losers Club has their initial showdown with the creature as children. In both the miniseries and movie duology, it serves as a gateway to Pennywise's true lair.
Miniseries[]
In IT (1990 miniseries), the cistern is a round, dome-like four-way intersection in the sewers. When the Losers first discover the cistern as kids, Stan Uris is chased by IT in the form of the Deadlights. The monster then floods the room with fog, trying to separate the Losers, before Pennywise appears and tries to eat Stan. Eddie Kaspbrak distracts the clown with his inhaler, claiming it to be battery acid, before Beverly Marsh shoots Pennywise in the forehead with a silver earring. The clown then leaps into a drain in the middle of the cistern and escapes to lick his wounds.
When the Losers return to the cistern as adults, they discover that plant roots have descended through the ceiling after years of neglect. The paper boat that Bill Denbrough made reappears, and IT takes the form of Georgie Denbrough to taunt Bill. When that fails, an image of Pennywise appears on the wall and warns the Losers to flee, lest they lose themselves in the Deadlights. Bill places the paper boat in a stream of water and it leads the Losers through a tunnel and into a cave, where it stops outside of Pennywise's lair.
2017 and 2019 movies[]
In IT: Chapter One, the cistern is a massive, round room in the sewers underneath the Well House. The centerpiece of the cistern is a massive pile of toys from Pennywise's victims over the years, and at the base is a circus wagon with the clown's name and visage on the side. Many of Pennywise's victims literally float around the top of the toy pile. At the top of the room is a large circular drain opening that could possibly lead out of the sewers. After Pennywise captures and Deadlights Beverly, the Losers rescue her, and IT takes the form of Georgie to taunt Bill. Not fooled, Bill shoots "Georgie" with Mike Hanlon's bolt gun, and he transforms into the clown. The Losers pummel Pennywise with various blunt weapons until he staggers and falls into a large hole.
In IT: Chapter Two, the cistern is flooded and the toy pile and circus wagon have degraded into a ribcage. After facing IT in the form of Mrs. Kersh, the Losers discover that there is a hatch on the floor of the circus wagon that leads to Pennywise's lair. When Pennywise is killed, his lair, the cistern, and the Well House collapse into the ground.