The Waterfall Room is a now removed secret area localized close to the Soccer Field and the tunnel. In between these two locations, there is a waterfall. When a player walks through it, they will find the secret room.
This secret location was added in the update v2.0.5.
Previously, there was a red button to the left of the disk that, when clicked, would activate a cutscene where the camera pans to the Sewers and zooms in on the grate, where the vampiric eyes inside squint. The button has since been covered up by a rock.
Inside of the room.
Pot Platinum, TheGuestToBlame and Tom AU standing next to a disk.