Stephens had himself lowered into the chaltun (underground cistern) in front of this building. He writes: "The chamber was entirely different in shape from those I had seen before. The latter were circular, and had dome-shaped ceilings. This had parallel walls and the triangular-arched ceiling; in fact, it was in shape exactly like the apartments above ground. It was eleven feet long, seven wide, and ten high to the centre of the arch. The walls and ceiling were plastered, and the floor was of cement, all hard and in a good state of preservation. A centipede was the only tenant after the evasion of the iguana.
This chamber was formed in the roof of the lower building. That building contained two corridors, and we had always supposed that the great interval between the arches of the parallel corridors was a solid mass of masonry. The discovery of this chamber brought to light a new feature in the construction of these buildings. Whether the other roofs, or any of them, contained chambers, it is impossible to say. Not suspecting anything of the kind, we had made no search for them, and they may exist, but with the holes covered up and hidden by the growth and decay of vegetation."
(Stephens, vol 2, p. 36-38)