The sewer in a location found in Goat City Bay.
The sewer can be found near the carnival on the ocean side of a bridge. It looks like a drainage pipe.
Upon entering the sewer, the player will immediately unlock the "Repulsive Goat" mutation. A small circular door can be found farther down the vent.
Entering the doorway will reveal several creatures green in color dancing in a small room, resembling the Teenage Mutant Ninja Turtles, but instead of turtles, their heads look like shrek with a mask. If the player enters the room, the creatures will usually attack, but other times may keep dancing.
The Deadgoa7 mutation can also make the creatures keep dancing, allowing the player to see the room.