Chuck Vanlue checks his handheld Global Positioning System (GPS) unit and slowly plods through the thick blackberry vines and stinging nettles that blanket the area. According to the unit's electronic distance gauge, he is within twenty feet of his destination. It is time to get serious.

The coordinates 43° N, 123° W mark his location: a small patch of woods bordering a rest area outside Cottage Grove, Oregon. Just beyond the wooded area, travelers inspect maps against the hoods of their cars and road–weary children scatter across the green lawn, tossing balls and Frisbees. A middle–aged man and woman walk an inquisitive black dog that sniffs at the tall grasses bordering the small patch of forest.

Cars zoom past on the nearby interstate as Vanlue crouches to overturn mossy logs and flat rocks. The dog walkers pause and eye Vanlue. He notices and rises, casually scratching his shaved head as if lost on a hike in the forest. "The trick is not to let people see you doing this," he says in a low voice. "They start to wonder what ' s over here, and they come and ruin everything."

"It's a crazy underworld....I'm constantly finding new places to go."

Vanlue, known online as "Seal Rock George," hardly looks the covert type — dressed in a spotless white t–shirt and tan cargo shorts. With an easy smile and friendly demeanor, he resembles Garth Brooks more than James Bond.

As the dog walkers lose interest and walk away, Vanlue carefully prods a stack of logs and bark. The pile is too ordered, too perfect. He digs through the wood and removes a plastic Folgers coffee container wrapped in camouflage duct tape. "There it is," he says, shaking the container. "There's the treasure." Grinning, he unscrews the lid and exposes the contents within: one miniature Barbie doll, a crumpled Pampers diaper, a pair of dice and a tiny notebook sealed in a sandwich bag.

Vanlue has just unearthed the latest of his some 400 geocaches. In this high–tech version of hide–and–seek, aptly dubbed geocaching, participants hide small containers filled with knick–knack fortunes, and then post the geographical coordinates of the caches on Internet forums. Anyone can hide — and find — a cache, provided they sign the logbook in the container and hide it back in the same spot.

As soon as a new cache is posted, fervent cachers hit the trail in search of treasure. The first to log a find earns bragging rights; competitors banter in online forums over who "got firsties."

Geocaching became possible in May of 2000 when the Clinton administration lifted a government–imposed GPS signal degradation. Until that time, in order to protect military communication, the government scrambled satellite signals for commercial GPS units — rendering the units accurate to only one hundred yards. When the signal degradation was lifted, commercial units became accurate to within twenty feet.

Two days after the signals were unscrambled, Seattle resident Dave Ulmer hid a container of goodies outside of Portland, Oregon for his friends to find using their GPS units. Following the expedition, Ulmer's friend started an online newsgroup that launched the spread of geocaching. Four years later, nearly 100,000 cache sites have emerged in all fifty states and in more than 200 countries.

Geocachers can now find treasures in nearly all types of terrain, including urban caches hidden in city parks and around buildings. More adventurous cachers leave containers on cliff sides, on the tops of mountains, in the ocean and at the bottoms of lakes. Conventional cachers must demonstrate creativity and cunning to evade outsiders and challenge the experienced. Vanlue says local terror "Evil Jim" is notorious for gluing fake moss and twigs to cache containers.

The expansion of the phenomenon has also created some confusion. In 2003, a farmer in Ellensburg, Washington called the U.S. Army bomb squad when he unearthed a suspicious tubular container in a roadway tunnel. After three hours of investigation, the squad exposed the contents: birthday–themed toys and knick–knacks. Due to concerns about national security and litter, the National Park Service has forbidden geocaching in its parks. In areas where it is still allowed, geocachers are advised to label their containers.

As the hobby spreads, the types of caches have become more creative. On a business trip to Ireland, Vanlue hid a "travel bug" (trinkets marked with serial numbers and tracked online) that later turned up in Maine. One Web site documents the migration of hundreds of plastic monkeys as they travel the globe from cache to cache. Hide–and–seek book clubs allow readers to swap literature through caches.

For cacher Jay Fox, the pursuit is an excuse to have fun outside with his family. Fox, along with his wife, son and two daughters, comprise the "Family o' Foxes" team that has found nearly 500 caches since 2002.

"Before my first time, I couldn't imagine why anyone would walk around the woods looking for a box of junk," says Jay's wife, Amy. "But after our friends took us out, our family was hooked."

After a few wrong turns in their search for the "Stuck Between a Rock and a Hard Place" cache, the family zeros in on the treasure. The Fox's youngest daughter, four–year–old Madeline, seems the least interested in the search as the family scours a fern–covered hillside. She moseys and picks flowers, but soon uncovers a Tupperware box wrapped in a black garbage bag, jammed between two moss–covered rocks. As she removes a particularly enchanting necklace from the cache and replaces it with a bouncy ball, her exhilaration reveals a greater purpose to the game than the modest exchange: youthful curiosity and adventure.

Vanlue bubbles with his own child–like enthusiasm as he drives his shiny, black SUV adorned with a large geocaching sticker on the back window. "It's not about prizes," Vanlue explains. "I hide the stuff I find cleaning out my house and garage." He constantly points out specific landmarks — road signs, fallen trees, boulders — along the way that mark caches he has already found.

Every weekend, his obsession leads him to new, unvisited locations. "It ' s a crazy underworld," he says. "I've lived in Eugene for fifteen years and I ' m constantly finding new places to go. Within a ten–mile radius, there are about 250 caches."

Vanlue's latest hunt for the "Turtle Pond" cache begins near a sewage treatment plant. On the hunt, he runs into a retired couple — known online as "GlenMart. " Though they communicate frequently online, he has met the couple only a few times on the trails.

But as "Mart," a short woman in a sweatshirt and a neon–pink baseball cap, hops out of her car, the bond formed online becomes immediately apparent. She laughs and promises to be the first to find the latest cache that Vanlue has hidden. Meanwhile, the three combine forces and agree to explore "Turtle Pond" together.

Edging past the sewage tanks and into the forest, the trio compares their most recent expeditions. GPS units are drawn and maps are consulted as the chatter settles on the current cache. Just beyond the rancid sewage tanks, wooded trails lead to a pristine park area and a small pond. Vanlue leads the way down the trail, pointing in excitement, as if the treasure hunt at hand is his very first.