The Mount Prospect Reservoir at Grand Army Plaza, circa 1900. The reservoir was built in the late 1850s at the second highest point in Brooklyn and supplied water for the borough before consolidation into greater New York City. Once Brooklyn became part of NYC they no longer needed their own water system and in 1911 the reservoir was filled and construction began on a new Beaux-Arts public library that would rival the one being built in Manhattan on 42nd and 5th at the site of the former Croton Reservoir.