ERIC BOLSTRIDGE likes to check on his hidden treasure at least once a day.

"Every time I leave for home, I glance over to see if anyone is there," says Bolstridge, 27, who lives in Burke and works near the Arlington County Courthouse.

Bolstridge's treasure -- four state quarters in a magnetic hide-a-key box -- isn't the kind of thing that would get Indiana Jones out of bed in the morning. But to Bolstridge and other geocachers, the reward is in the hiding and finding.

Geocaching is an adventure game using hand-held Global Positioning System devices, which tell you your longitude and latitude to within 20 feet or so with the help of satellite signals. The technology was developed by the military in the 1960s; geocaching started in May 2000, the month civilians were granted the same access to GPS signals as the military. GPS devices, which can run from $100 to $1,000, have become popular with hikers and campers. Geocachers hide "caches" -- usually weatherproof containers, such as an ammunition box or Tupperware bowl -- and send the location coordinates to www.geocaching.com, the game's leading Web site. Site administrators make sure the area is safe and environmentally sound (geocaching is not allowed on National Park Service land, nor should anyone try to hide an ammo box next to a federal building), and then post the coordinates online.

The site lists more than 1,600 caches hidden within 50 miles of downtown Washington.

Caches usually contain such items as small toys, party favors or disposable cameras and almost invariably include a logbook and pen or pencil.

Once a cache is posted, other geocachers get the coordinates and enter them into a GPS device, which then tells them how far they are from it and which direction they need to go to reach it. It's like an electronic version of someone telling you, "You're getting warmer." Although it might sound simple, the trick is in spotting and retrieving the cache, whether it's hidden in a remote marsh or at a busy street corner.

Each cache is rated 1 (easy) through 5 (difficult) as well as 1 through 5 for terrain rating, 1 being stroller and wheelchair accessible, 5 requiring climbing gear. Once a cache is found, cachers are encouraged to take an item, leave something new (these tokens often become cachers' calling cards) and sign the logbook.

"Holy Jumping Jiminy!" reads an entry in the logbook of one of John Grace's caches. "Fifth times a charm."

The cache -- an ammo box whose contents include toy soldiers, a balsa-wood glider and dinosaur-print pencils -- is one of several Grace hid in the 68-acre Audubon Rust Sanctuary in Leesburg. It took him six months to convince land managers that geocaching would be a good way to attract visitors.

"I wanted to help share places I like with people who might never find them," Grace says.

Without giving away the location, the path to the cache took Grace past a family of deer and a heron preening in a pond.

"Scared off four ducks on the approach," reads another log entry. "What a paradise."

Grace, 39, a flight attendant who lives in Leesburg, began geocaching in 2004 and is one of 100 or so core cachers in Northern Virginia. He organized a geocaching event in early October in Ashburn.

"It's pretty addictive," Grace says, explaining the appeal of geocaching. "There's a thrill in searching for things and finding hidden treasures."

The game has grown in popularity in the area in the last year or so, says Robert Hicks, a spokesman for the Maryland Geocaching Society. The group, founded in 2002, has more than 1,000 members. Grace, who has attended geocaching events across the country, considers the society the best state group he has encountered.

"MGS offers a place for geocachers to get together and discuss ideas, issues, and just feel like part of the community," Hicks wrote in an e-mail interview. "We have a pretty good Web presence, and MGS has been very successful at bringing folks from every corner of Maryland together."

Bolstridge took up geocaching in March while going on walks with his wife, Tris, and newborn son Mark. For him, the appeal in finding a cache comes from knowing a secret.

"It's fun knowing every time you walk by it again. You say to yourself, 'Aha!' " Bolstridge says.

Soon he was planning different routes to and from work based on their proximity to caches. After he got some experience, he started hiding his own.

Urban caches -- such as the "sbux" caches hidden near Starbucks coffee shops across the country -- present special challenges. If a cache is hidden, for instance, in the bushes outside a building on a busy street, how does a cacher get to it without giving it away, short of loitering for a couple of hours?

"You can't get it until it's clear, and you can't wait until it's clear," Bolstridge says.

Because geocaching depends so much on participants acting honorably -- by not moving, defacing or revealing the location of a cache -- Bolstridge is a little worried that the activity's growing popularity could also be its undoing.

"Be sneaky when you find a cache, so you don't give it away," Bolstridge says.

Caches typically contain a short, laminated description of geocaching in case hikers, road crews, maintenance workers or other non-geocachers stumble upon them.

There are several variations on geocaching, such as multicaches, which take players across several stages, revealing new coordinates at each leg, and virtual caches, which end at a particular statue or monument.

There's no scoring or winners or losers in geocaching. Goals are self-imposed: Some participants want to accumulate as many caches as they can, others want to find caches in all 50 states or all the caches with a level-5 difficulty.

One of the most challenging caches is the "Blood and Guts" listing in Northern Virginia, which was established in 2001. Grace was on a team of six people that took almost two months to finish all five stages.

A note on the listing at www.geocaching.com gives an idea of its scope: "Warning, this adventure is not for the Faint of Heart. Candidates for this quest must know local history, New World Order conspiracy lore & rumor, cryptology, and celestial navigation. Determination, persistence and a bit of nerve will also be needed."

Bolstridge says newcomers should search for level-1 geocaches before trying something more difficult.

"Don't even bother trying a 4 before you're ready -- you won't find it. . . . Every cache you find makes finding the next one easier," he says. "There are very few unique caches out there. About half of the caches hidden in the woods are either hidden under a pile of sticks, next to a fallen log, are in a stump or in a hollow tree. Experience will teach you where caches are typically hidden, and you can find them much easier once you know where to look."

RESOURCES

GEOCACHING -- www.geocaching.com.

MARYLAND GEOCACHING SOCIETY -- www.mdgps.org.

HOW GPS RECEIVERS WORK -- www.howstuffworks.com/gps.htm.

BUXLEY'S GEOCACHING FAQ -- www.brillig.com/geocaching/faq.shtml.

John Grace leads geocachers, including Natalie Zimmerman, along Goose Creek in Leesburg.Cache coordinates are plugged into Global Positioning System devices.