Two days after President Clinton granted civilians access to a Defense Department network of satellites in 2000, a man hid a notebook inside a watertight ammunition container in the woods near Portland, Ore., and posted the longitude and latitude coordinates on the Web.

Before you could say Lewis and Clark, the game -- or sport -- of geocaching was born. Players around the world were investing in a hot, new high-tech toy, a hand-held Global Positioning System receiver, checking clues on the Internet and searching for the hidden container and similar treasures.

According to www.geocaching.com, that single Web posting has morphed into a worldwide scavenger hunt encompassing more than 127,000 "caches" in 210 countries, including dozens in Prince William County. Some hiding places are almost predictable. Others would frustrate Indiana Jones. Most of the stash in the cache is straight out of a cereal box or the Dollar Store, so the thrill of the hunt isn't about searching for the Holy Grail. It's about the thrill of the hunt.

"You get so used to looking at that indicator that you get to your destination and realize you don't know where your car is," said Ray Pfaff, a software engineer from Chantilly. He is speaking from experience.

The rules of geocaching are simple. ("Geo" comes from geography; a "cache" is hunting and camping lingo for a hiding place for provisions or in computer terms refers to information stored in memory.) A waterproof container, such as Tupperware, an ammunition can or even a film canister, is filled with trinkets or items to trade and a logbook for recording visits. It is hidden but can't be buried. The information is registered on the official Web site. Geocachers download longitude and latitude coordinates into their GPS receivers after locating caches by Zip code, state or country, and they're off and running. Or climbing. Or swimming.

"Within 100 miles of here, there are more than 2,600 caches," said Bill Tobalske, a retired Air Force chief master sergeant from Reston whose geocaching "handle" (many geocachers use an alias) is 2Wheel'in.

"We tease him that he can't spell," said his wife, Bonnie, a Fairfax County firefighter and paramedic who goes by the handle MooseMaMa. "And he repairs bicycles now that he's retired."

The Tobalskes went to Sterling recently to look for one of two caches hidden in Claude Moore Park.

One cache, Claude's Vestals Gap Cache, belongs to the park, said park manager Pam Sheets, a new and enthusiastic geocacher who goes by the handle "iplay."

"It's a mishmosh of items from Claude Moore Park and the Loudoun Heritage Farm Museum, including pencils, key chains, rubber spiders, rubber snakes -- even a rubber cow," she said. "And a logbook, of course."

The park's other cache, a private one called "Cat's Meow," is filled with toys for cat lovers. Cache owners who want to use public land must get permission, Sheets said, so they aren't placed where people might stomp on wildflowers or disturb endangered species, for example.

"The way people are hiding things is becoming more interesting," Bonnie Tobalske said. "Some people leave a cryptic message that you have to figure out."

One person took a series of two-sequence words from the Declaration of Independence after numbering all of the words.

"You had to go back and find out which number words were selected in order to obtain the longitude and latitude," Bill Tobalske said. "Some people are demonic," Bonnie added.

The Tobalskes decided to search for Cat's Meow. Bill knew where it was, but for Bonnie, it was her 101st cache. Bill plotted the location coordinates, known as a "waypoint," on a sophisticated color GPS receiver.

"The trick to finding the cache is to follow the direction of the arrow on the screen -- and to keep moving -- or else the arrow disappears when the tracking satellites can no longer ascertain the direction of travel," he said.

A GPS, however, knows only how close the cache is as the crow flies -- that is, in a straight line -- meaning it has no clue about altitude or terrain. To avoid surprises such as the fact that you need to cross the Potomac River to find your cache, some geocachers purchase topographical maps. And for the ultra-ultra-serious enthusiast, the information is available on CD-ROM, as long as the GPS has computer-download capability.

"Remember the direction of that arrow when you get within 30 feet of the cache," Bill Tobalske said. "Because after that, you rely on your wits to find it. The GPS can't help when you're that close."

"I always look for 'UPS,' " Bonnie said. "That's an Unusual Pile of Sticks. Or tromped-down leaves."

They're not all tangible, either. Ray Pfaff placed what is known as a "virtual cache" at a Civil War marker commemorating the Battle of Chantilly in the Second Battle of Manassas. Anyone who finds the marker in a small park at the corner of Monument Drive and West Ox Road in western Fairfax County reports his or her visit on the geocaching.com Web site.

"The reason I find it so important is if the Confederate forces had been just a little bit faster, they could have stopped the Union forces from retreating back to D.C. and made them surrender," Pfaff said. "That would have left D.C. more or less undefended and could have changed the whole outcome of the war."

"It's a good way to learn about history," agreed Scott Allen of Herndon, a geocacher whose work for the Air Force involves combat search and rescue. "It's educational and interesting." Allen often goes geocaching with his 10-year-old daughter, Samantha, and his wife, Susan.

The Cat's Meow, which has been around since Aug. 21, 2002, turned out to be an ammo can containing a logbook, pen, several cat toys, a small truck, assorted odds and ends and what was generally determined to be Mr. Potato Head. Bonnie Tobalske removed the truck, signed the logbook and left two yellow erasers in the shape of a firefighter's helmet and a fire hydrant.

"Geocaching takes me to places I probably wouldn't go to," she said. "That's my goal in life -- to see as much as I possibly can."

Ray Pfaff and Samantha Allen use their Global Positioning System units to search for a cache hidden somewhere in Claude Moore Park in Sterling.