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The McMillan Sand Filtration Site is one of Washington, DC’s most conspicuous mysteries: a fenced-off 25-acre grassy plain just over 2 miles north of Capitol Hill, marked by rows of tall concrete cylinders clothed in overgrown ferns. Unknown to the thousands of commuters and residents that pass by its rusted gates daily, below this sprawling parcel of land lies a series of vast underground caverns built in the early 20th century by the Army Corps of Engineers as a natural purification facility for DC’s turbid water supply.
Sealed to the public at the onset of World War II, the park above the filtration cells has been inaccessible ever since. Now, almost 30 years since its official closure, the site – a curious holdout among DC’s recent wave of rapid urban development – has become the subject of a widespread debate over its future use.