The stone structure is a 157 foot vertical control shaft connected to Tunnel No. 1, used to open and close the flow of water running through the tunnel pipes. Two known accidents occurred here: in 1874, Tunnel No. 1 collapsed on two Chinese workers, causing an uproar in the camp, and in 1875, water forced down the control shaft to dislodge debris washed four Chinese laborers out of the tunnel, though all four survived their injuries. The San Leandro Plaindealer in 1874 portrayed the hill as a busy work site filled with the glare of the forge, the ring of the smith's hammer, the noise of the wheelwright's saw, and men hurrying to and fro as a grand site was being achieved. The first superintendent, Frank Boardman, lived in a cottage on the knoll, and his father William Boardman, a former Alameda County supervisor and the dam's chief engineer, created many of its building diagrams and drew the plans for the two water supply tunnels running through the hill.