We felt very lucky to be able to visit the McMillan Sand Filtration Site, where the District's water was filtered from 1905 until 1985. The huge underground complex was filled with sand that filtered the city's water. The sand had to be removed frequently and washed, then dried and returned to the system. The green space on top of the complex was landscaped by Frederick Law Olmsted and looks like a modern green roof despite its age. It is a fascinating site that will eventually be redeveloped. But for now it makes for an amazing day of urban exploration when it infrequently opens to the public. It was cold and sleety but we're very glad we made the (short) trip!