Deep below the nation's capital, massive tunnels are being built to save Washington's rivers—the Potomac and the Anacostia—from severe water pollution.

To get to where one of the tunnels is currently being dug, you must take a metal cage elevator 100 feet (30 meters) below the Blue Plains water treatment plant in southwestern D.C. The concrete-lined shaft is dry now, but eventually storm water will rush through.

At the bottom, visitors board a maintenance vehicle. The truck-size transport slides along train tracks for half an hour. Belts, wires, and tubes dangle into the tunnel. Visitors are asked to keep hands and feet inside the moving transport at all times.

It's hot, dark, and slightly musty, with sporadic lighting along the walls. Small puddles