BALDWIN RESERVOIR of the Baldwin Filtration Plant and Fairmount Pumping Station is believed to be one of the largest covered reservoirs in the world. Completed in 1925 to store the treated water from the filtration plant before distribution to the service area, the underground clearwater reservoir is an engineering marvel. It is 500' wide, 1,000' long, and 35' deep and has a capacity of 135 million gallons. It is roofed by a thin groin-vaulted concrete slab supported by 1,196 columns, which are 30" in diameter and spaced 18' apart. Before the reservoir was filled, the underground space had the appearance of a vast and mysterious cathedral. The roof is covered with soil, making a park-like lawn adjacent to the filtration plant. The plant itself is a symmetrical Palladian structure 750' long, which dominates the commanding site at the summit of Fairhill Rd.