The Pit of Refuse is located inside Temple of Atal'Hakkar. It is full of bog beasts, oozes, maggots, and worms. It is located directly beneath the Pit of Sacrifice and is accessed either by a long winding stairwell leading from the rightmost passage upon entering through the instance portal or by jumping down from one of the six Hall of Serpents' ledges lining the room. Dropping through the central pit from above will result in a swift death from falling.
Upon pressing the snake statues on those ledges in the proper order, the dire troll boss Atal'alarion spawns in the middle of the pit. The altar showing the proper order of the statues sits at the top of the staircase leading down onto the water-covered pit floor.