Across from Children's Hospital is a field with a bunch of concrete silo-looking structures. It is all overgrown with grass now and has a fence around the entire field. What the heck is that place and what was it used for?

Thomas R. DeYulia Jr., Alexandria

If you've ever driven on North Capitol Street or Michigan Avenue, you've seen this odd array. A dozen or so cylindrical structures rise from a field. It looks like the remains of a launch pad for Victorian spaceships or a planned community for hobbits or a gigantic game of Whack-a-Mole or a piece of environmental art.

What is it really?

"Remember [how] we keep hearing that the vice president is at an undisclosed location?" said Tom Jacobus. "He's moved between those silos."

He's joking. Tom is general manager of the Washington Aqueduct. If you know your Latin, you know that "aque" is derived from the word for water. Of course, if you know your English, you already know what "aqueduct" means.

And water is the key to understanding what those buildings are -- or were.

The silo-studded field is next to McMillan Reservoir, one of the holding areas for the city's drinking water supply. From about 1900 to 1985, the silos were an integral part of what's known as a slow sand filtration water treatment plant. Untreated Potomac water, fresh from Great Falls, arrived at the plant and was pumped over 26 massive rectangular concrete underground filters. The main ingredient in the filters was sand.

"Particles in the water got filtered [out], and clean water would then flow to the bottom. . . . It was really very modern at its time," Tom said.

But if all this was happening in underground chambers, why those cylinders jutting above the ground?

Because when you've got a slow sand filtration system, it almost goes without saying that you go through a lot of sand. Some of it washes away. All of if needs to be cleaned regularly, since junk from the water -- the very stuff you're filtering out -- starts to collect in the sand.

So clean sand was stored in the silos.

You slid a cart up to a silo "and filled it up with sand, then took the cart over to an opening and put the sand right in the filters," Tom said.

Dirty sand from the filters was cleaned regularly. "These sand filters would be taken out for wash -- they'd be in service for almost a whole year," said Raymond Moton, the Washington Aqueduct's water treatment unit supervisor. "The schmutzdecke on it had to be washed out."

You read right: schmutzdecke. If you can say one thing about today's column, it's that you learned a word you probably didn't know. Schmutzdecke -- or, occasionally, smutsdecker -- is what water treatment folks call the crusty layer of dirt that cakes on top of the sand. When that happens, the water can't filter through as well.

So, Raymond explained, workers "would take the big rakes, like on a tractor, over the sand to break up the schmutzdecke." The sand in each filter would also be removed and stored in a nearby silo as the filter was washed out more completely.

The grounds of the filtration plant were designed by Frederick Law Olmsted Jr., son of the famed landscape architect and an esteemed artist in his own right, who did much to contribute to the District's architectural renaissance in the early 20th century. Olmsted's design included interesting geometric pathways among the silos. The whole thing used to be a public park, but it was closed to visitors permanently for security reasons during World War II.

In 1985, a rapid sand filtration system was built on the other side of First Street NW, near McMillan Reservoir.

That operation requires less space, so the old slow sand plant was closed.

The District government now owns the 25-acre site and is hoping the property will be revitalized, with an assortment of attractions that include open space, retail offerings and a museum to the site's historic past.

When Worlds Collide

Last week, Answer Man said definitively that the thrum of cicadas was not used as a sound effect in "Invaders From Mars." That brought a torrent of e-mail that said, basically: "Of course not, you idiot. The cicada sound was in 'The War of the Worlds,' which was also released in the Brood X year of 1953."

Other readers said they'd heard cicadas in "Them" and "Earth vs. the Flying Saucers."

What is clear is that "chorusing" cicadas remind people of something out of this world. Ironic, given that cicadas are very much of this one.

The Giving Tree

Here's an easy answer to the question, How can I help a needy kid this summer? Simply donate to our Send a Kid to Camp campaign.

Here's how to contribute: Make a check or money order payable to "Send a Kid to Camp" and mail it to: Attention, Lockbox, Department 0500, Washington, D.C. 20073-0500.

To contribute online, go to www.washingtonpost.com/camp. Click on the icon that says, "Make Your Tax-Deductible Donation."

To contribute by phone with Visa or MasterCard, call Post-Haste at 301-313-2200 on a touch-tone phone. Then punch in KIDS, or 5437, and follow the instructions.

Researcher Alex MacCallum contributed to this report. Send your queries to answerman@washpost.com. Or write John Kelly, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071.