Eric Fidler answers a question that’s been gnawing on my mind ever since my last ride down North Capitol Street from the Catholic campus: what’s with the brick silos and open fields between Michigan Avenue, N.W., and Channing Street, N.W.? An abandoned Nike base? Sequestration of toxic waste from the big domed building at the start of North Cap? The Undisclosed Location? (It looks even more hush-hush on Google Maps.) No, it’s the remains of the McMillan Sand Filtration Plant, which originally purified the city’s drinking water.