During the final mission, "The Beginning of the End", you will enter an Eridian structure with a large circular chamber with elevators rising up through the force field on the floor. Notice that one of the elevators in this room does not have a force field at the top. Use it to rise up to a new room that has blocks, lava, and pipes that are reminiscent of the Mario series. You can also find a few chests in this room.