On a "Naturalist's Tour of Arlington" our group walked to a strange concrete platform in the Potomac River, upstream maybe a quarter-mile from Chain Bridge. We accessed it from the canal side. It's quite large and seems to have trap doors leading down to who knows what underneath. Guesses as to its purpose ranged from "an abandoned helicopter landing pad" to "this must be where Cheney hid on 9/11." Can you find out anything about this mysterious construction?

Ann Kurzius, Arlington

There are two new signs on Chain Bridge -- one in each direction -- sharing what might be interesting information. However, unless you are a pedestrian or stuck in traffic at just the right place, you pass by too quickly to read it. In two trips across the bridge we have pieced together this much: "Potomac Gorge . . . This 15 mile . . . Roosevelt Island . . . concentration of globally-rare . . . communities in the nation." Can you please find out what this sign is all about and print its complete text?

Marilyn Taylor, Arlington

Are these questions related? Yes, indeedy, since both involve what's known as the Potomac Gorge, a little patch of our back yard that is literally crawling with interesting fauna and flora.

The Answer Family recently trekked to the aforementioned concrete structure and found the reader's description to be accurate. The area is reachable by a concrete road and comprises two installations: a large concrete pad about 60 feet square and a smaller concrete area overhanging the Potomac that is inset with five metal doors and bounded by low walls.

It reminded Answer Man of a sacrificial altar, but then, most things do.

In fact, it's an emergency pumping station, said Tom Jacobus, general manager of the Washington Aqueduct, which supplies most of the Washington area with water. (If you've ever seen "Chinatown," you know it always comes down to water.)

Let us travel back to 1966: The Beach Boys release "Pet Sounds," the Orioles win the World Series and the Washington area is locked in a severe drought. The Corps of Engineers is so concerned that someday there won't be enough H2O that it decides to build an emergency pumping station as far downriver as possible. It's right near where the Potomac becomes tidal, ensuring that there will always be water available.

The idea is that should things ever get really bad, five pumps would be installed in the five "doors" and powered by generators on the concrete pad. The pumps would suck water out of the river and push it to the Dalecarlia treatment plant.

But by the time the pumping station was completed in the early 1970s, it had been decided that the area's water woes would be dealt with in a different way, by building the Jennings-Randolph Reservoir on the West Virginia-Maryland border. The station instead was used to discharge the dirt and other solids that are left when water is cleaned at Dalecarlia.

New EPA rules stipulate that solids must be handled differently, so the station is once again being considered for emergency water intake.

But that's only part of the story. For the concrete road leading to the pumping station follows a cut in the landscape that was made in the 1920s as a tailrace for a now-shuttered hydroelectric power plant. The rock and rubble removed to build that tailrace was positioned downstream. When the Potomac runs high, this berm creates all kinds of nasty currents and eddies that disturb the environment.

Which brings us to those signs on Chain Bridge, which went up in September and read: "Potomac Gorge -- The 15 mile section of the Potomac River from above Great Falls to Theodore Roosevelt Island - has the highest concentration of globally-rare natural communities in the nation."

Answer Man finds that little hyphen in the middle of the sentence unfortunate. And the wording makes him wonder whether the rest of the country had been cut into 15-mile segments, measured against the Potomac Gorge and found inferior.

The signs probably should say, "one of the greatest concentrations of rare species" in the nation, said Stephanie Flack of the Nature Conservancy, one of the groups that lobbied for them. (The Palisades community, the Potomac Conservancy and the U.S. Geologic Survey were also involved.)

But there's no denying that the 10,000-acre gorge -- formed by the eroding powers of the river -- is amazingly biologically diverse. It's home to more than 1,400 species of plants (hairy wild petunia, sweet-scented Indian plantain, Virginia mallow. . .) and all manner of animals, from Pizzini's amphipod to 14 species of salamander. Many of the species, as the signs point out, are "globally rare," meaning they've been found in five or fewer places on the planet.

Why so rich? The Potomac Gorge is at the intersection of two geologic formations, the rocky Piedmont Plateau to the west and the Atlantic coastal plains to the east. It's a crossroad for countless critters. Because it is constantly scoured by floodwaters, it provides little nooks and crannies where animals and plants can get a foothold -- or a roothold.

Taken together, the pumping station and the signs symbolize the relationship we have with the Potomac. We count on it for life itself, tapping its waters to fill our taps. But it's good to be reminded that lots of other living things count on it, too.

Curious about something you've encountered while walking, hiking, sauntering, gallivanting or perambulating across the Washington area? Ask Answer Man: answerman@washpost.com.