Peering under rocks, stepping over stumps and knifing through overgrown brush, Brian Yonke inspects an urban jungle in his home town of Liberty, Mo., with near-surgical precision.

The thickly wooded area teems with ticks, spider webs and poison ivy. Near 100-degree heat causes sweat to drip from his chin in dime-size drops. He doesn't care; his $400 Global Positioning System device tells him there's something hiding here, and he and his wife, Carlin, aren't leaving until they find it.

The Yonkes are part of a growing group of modern-day Magellans, tenacious treasure hunters who scour the Earth in what amounts to a global game of hide-and-seek for adults. The Yonkes' passion is geocaching, a gadget-driven, Internet-assisted pastime that uses map coordinates and satellite signals to guide searchers to a world of hidden goodies.

"It's become an addiction," Carlin says.

In the last three years, the two have found more than 1,100 objects in more than 30 states. Like all geocachers, the Yonkes use handheld GPS devices (they own five) to literally point the way. They check the main geocaching Web site, www.geocaching.com, for "caches" to locate.

Although geocaching is the most popular form of adult hide-and-seek, it is not the only one. Letterboxing, which originated in England 150 years ago, is also gaining a following in the States.

More artistic than geocaching, letterboxing combines hiking with puzzle-solving and creating hand-carved, personal stamps. Visitors who find a letterbox mark the logbook inside with their stamp and use the stamp they find in the box to mark their journals as a memento of the discovery.

Letterboxing does not require a GPS device. Instead, seekers follow step-by-step clues found on the game's Web site, www.letterboxing.org, like a treasure map. Clues can be simple or devilishly complex. Hard-core letterboxers pride themselves on creating elaborate, challenging, even poetic clues. Many employ metaphors, symbolism and other literary devices. As a result, letterboxing is often as much a mental and artistic exercise as a physical one.

Wes Garrison, one of the primary Webmasters for the main North American letterboxing site, lives in Olathe, Kan. He and his girlfriend have carved 15 stamps; his favorite bears a beaver and a sunflower. He carved that stamp after meeting a letterboxer from Oregon and jointly placing a box locally with him representing Kansas and Oregon.

Geocachers and letterboxers don't always see eye to eye, but their passions are simply different ways of doing the same thing. Both activities combine mystery and adventure with exercise and a way of connecting with others.

Thompson Brown, a behavioral therapist outside of Chicago, said the games make perfect sense for today's increasingly detached world.

"Even as we isolate ourselves in our air-conditioned homes, glued to our TV and video games, we long for human interaction, and a way to say, 'I'm here. Notice me,' " he said. "As children we played hide-and-seek in our world as a way to have fun and interact with others. As adults we're no different. Our world has just gotten a lot bigger."

Brian Yonke simply loves the hunt. He stares at his GPS device as he and his wife cross a field and walk down a large hill. As he walks, the device shows him which way to go and even tells him how fast he's walking. He crosses a footbridge into the woods. The device says 143 feet to the target. Then 121 feet. Ninety feet.

"Almost there," he says.

Geocachers and media wags alike have called the hobby "the first new sport of the new millennium." Since a GPS user hid the first cache more than four years ago, the activity has exploded. And the Kansas City area is one of its main beneficiaries.

Garmin International, headquartered in Olathe, is one of the world's largest manufacturers of GPS devices. The company has sold more units in the last four years than in the previous 11, at least partly because of the hobby's growing popularity.

Jeremy Irish of Seattle, who runs the game's Web site, estimates that there are now more than 111,000 caches hidden around the globe in virtually every country on Earth. More than 90 percent of all caches are in the United States. Each week, 70,000 new logs involving geocaching activity are posted on the Web site.

It all started in May 2000, shortly after President Clinton signed an executive order that improved the accuracy of commercial GPS devices from about 300 feet to less than 20. Two days after Clinton's announcement, a man outside Portland, Ore., hid the first cache, and the sport has been growing ever since. GPS receivers, originally developed by the Defense Department for military use, are now produced in increasing numbers strictly for commercial use.

"In the last six to eight months we've introduced several GPS receivers that have a special geocaching mode," said Garmin spokesman Ted Gartner. "It allows hard-core geocachers to log their geocaching history -- where they've been, what they've found and when they found it."

The units range from $350 to $550.

Gartner, who takes his two young children geocaching, said, "It's a great way to get them away from the TV where they can learn something about the outdoors, about science, and find a cool cache loaded with a lot of trinkets that they can take home with them."

Usually caches contain a collection of small items such as keychains, McDonald's toys and erasers. But they've also been known to hold more valuable items, such as clothing or even cash. If you take something, geocaching etiquette requires that you leave something of equal value.

There are a few shared rules for both geocaching and letterboxing. No food or dangerous materials should be placed in boxes, personal property rights of landowners must be respected, and items should never be buried.

Leaving Their Stamps

Chris Fuller of Blue Springs, Mo., likes to search for things, too. But as far as geocaching is concerned, she doesn't know GPS from UPS.

She doesn't have to. Fuller is a letterboxer. She doesn't need anything but a brain and a good pair of shoes. Today, according to Letterboxing.org, there are about 10,000 letterboxes hidden across North America. Fuller finds all the clues she needs on the Web site. In the last year, the number of visitors to the Web site has jumped from 206,000 to more than a half-million.

Under a high sky in George Owens Nature Park in Independence, Mo., Fuller grabs her gear.

"Harry Truman Summer White House," she says, reading the name on the letterbox from her Web site printout. "Ready to read the clues?"

They're easy, straightforward. No metaphors or puzzles.

Go into the clearing, cross a bridge and walk down the Butterfly Trail until you get to a black locust tree. Fuller dutifully follows each clue, passing beavers and wild daisies along a forest trail until finally . . . she's lost. Wait. She never saw the black locust tree, and now the trail is splitting. She turns around and retraces her steps until she finally finds the tree. A little searching produces the letterbox, hidden at its base under a rock and a brick.

Inside she finds the placer's e-mail address, a journal and a hand-carved stamp bearing Harry Truman's likeness that says "Prez 1945-53."

She writes a message in the box's journal and then in hers before hiding the box again. She tries to use the stamp pad inside the box to mark her log book, but it's dried out. Next time she'll bring her own pad.

Letterboxing is just pure fun, she said, especially when the clues are pithy and cryptic.

"The first one we found was in Saugatuck, Mich.," she said. "The person who wrote the clues kept referring to fallen soldiers. Turns out she was referring to huge maple trees that had fallen down."

Ryan and Allison Day of Olathe have been letterboxing with their two young children for about a year. They've found 35 letterboxes and placed about 25 in four states, and have connected with searchers as far away as Ontario.

Letterboxing brings out their creativity. They once placed a pair of boxes with clues based on Aesop's fable of "The Hare and the Tortoise."

"We took the original fable and rewrote it so as you read the story you can see the clues to where you are going," Ryan said. "It was pretty tricky, but it's satisfying to know that you are adding a little extra treat to the person who is finding it."

Still, Ryan enjoys the stamps the most. He loves to search for intricately carved ones.

"There's a number of letterboxes in north central Kansas near Russell that have a really interesting stamp," he said. "They're placed by a man known as Der Mad Stamper. Each one of them depicts a nature scene at an area lake. We made a special vacation just to find that series."

It's not fun when you lose your stamps. One time geocachers looking for caches found his hand-carved letterbox fish stamp. Geocachers, who don't use stamps, mistook the stamp for a geocaching trinket, Ryan surmised.

"They thought it was a geocache," he said. "They took my stamp and left behind a bandanna. My children have never forgiven them for that."

Getting and Giving

Brian Yonke is getting really close now.

"Bearing oh-four-two," he calls, meaning 42 degrees east.

Halfway down a weedy path the GPS receiver beeps.

"It's about 16 to 18 feet straight ahead," he said.

The GPS gets him to the general vicinity. From there he has to search. After several minutes, his eyes light up as he points at the base of a tree. He reaches into a hole and pulls out an old peanut butter jar painted pink with black spots, like a ladybug.

Inside, along with a logbook, are a letter opener, wooden nickel and a fish-shaped key chain. Carlin digs into the pocket of her brown backpack and pulls out a small rubber gecko that she hands to Brian to place in the jar. He removes some lip balm from the jar (it could attract animals), signs the book and hides the jar.

He smiles, wipes the sweat from his brow and starts back down the weedy trail.

Mission accomplished.

Using a Global Positioning System device, below, Brian Yonke of Liberty, Mo., is among those playing a modern game of hide-and-seek.Brian Yonke signs the log found inside a container he located in the woods.