Halfway through their constitutionals, dog walkers, trail trippers, and other visitors to the Potomac Regional Overlook Park in Arlington encounter this huge listing concrete structure that spans the bed of Donaldson Run.

The wall, which stretches 50 yards across the Potomac tributary, crumbles to reveal twisted Rebar, steel rods bested by the weight of water and sand. A rusted iron winch sitting atop the rampart seems designed to operate a small floodgate near the base. Erosion, which has clogged the opening with stones and silt, renders the mechanism useless.

A sand-filled room adjoins the wall’s downstream side. At first glance, the chamber is reminiscent of the sweat boxes omnipresent in WWII prison camps. But the hut’s accoutrements—empty cans of Milwaukee’s Best and thrasher graffiti—denote a prison of the mind instead.

Now relegated to teen-age love den and mosquito incubator, this edifice was once so vital that some poor fools hauled tons of cement and steel up (or down) a half-mile of stream bed.

That doesn’t make it Stonehenge, but it does make you wonder.

A Washington City Paper T-shirt will be awarded to the reader who constructs the best explanation for the overlook’s obstruction. Inspired answers will appear in next week’s edition if they reach us by Monday. Submit your description, or suggest objects that have been puzzling you, by writing to: Mysteries, Washington City Paper, 2390 Champlain St. NW, Washington, DC 20009. Our fax number is (202) 462-8323, or e-mail us at Mysterieswashcp.com. No phone calls, please.