As the numbers drop, I move faster. Dried twigs snap, leaves rustle. I'm on the hunt this late autumn morn. The display on my GPS receiver shows 517 . . . 453 . . . 326. Although I've been hiking for several miles, my body urges me to run, but I must resist, keeping my senses alert to avoid missing the quarry. The count drops, drops-well under 200 feet-then suddenly starts to climb. I freeze and look behind me. I see a small opening in the trees and brush. Entering, I follow a minor trail into a clearing, once the foundation of a cabin with a spectacular lake view. Just inside the ring of stone, two large pieces of birch bark nestled below one of the rocks look suspicious. I move them and find it: a geocache. A plastic container placed in a hollow contains knickknacks. Opening it, I place a plastic pen and take a pin-my trophy. The container goes back into its corner and the bark into place.

This is the new activity of geocaching: part treasure hunt, part outdoor exploration. It owes its birth in 2000 to human ingenuity, the Internet, and Global Positioning System (GPS) technology. The concept is simple. One person puts together a collection of things-toys, mementos, trinkets-and places them in a container, called the cache, takes a reading of its position with a GPS device, then posts the location numbers on a Web site. Someone else looks up the location, finds the cache, takes one item from the collection, and replaces it with another.

It sounds easy; you can choose to take a short walk down a well-manicured trail in a park to a cache that you discover after a few minutes of looking. Or you can select a hunt that involves hiking into mountains or remote wilderness and searching for an hour-or far longer-to find the container. You can find caches tucked away in urban nooks and crannies, or on cliff faces that require the seeker to rappel down to reach the hiding place. Geocaching can be as domestic as the wooded area just a five-minute walk from your home, or as exotic as scuba diving 100 feet down off the coast of Bimini to find the final clue to a cache. You can hunt for dime-store knickknacks and toys from fastfood restaurants, or participate in contests for thousands of dollars, or just find caches for the bragging rights. Go for solo jaunts, or take along others. Enjoy the health benefits of physical activity, the pleasure of play, the aesthetic delight in the natural world, and the challenge of a puzzle, all at the same time.

The experience is as varied as the possibilities, and can lead to moments stolen out of time. A few miles from where I live, on a street I sometimes take as a shortcut, a single quiet road heads off to the side. Over the years, I had never wondered what lay down there. In fact, its existence barely registered with me. Early in my geocaching experience, one nearby cache had me circling a large undeveloped area. Finally sitting down with an online map, I noticed this road and how it seemed to extend toward the cache location.

Taking that turn for the first time, I found myself pulling into a dead end where a plaque for a memorial forest stood. It was overcast, a little drizzly, and wet from previous rain. I marched in, following the readings on my GPS unit, kept to a trail, past an insignificant offshoot, and ever closer to the final spot. It seemed to be off the trail on the border of a marsh. Slowly I dodged and weaved past bramble, which grows faster than rumors where I live. Five feet, ten feet, twenty: progress was slow, and I had yet to learn the lesson that when a shortcut seems advisable, the searcher is usually delusional. A foot in the damp muck finally convinced me, and I looked around, then noticed that just beyond bramble thick enough to audition for a production of Sleeping Beauty there seemed to be another trail: that blasted offshoot that I had ignored.

Being no prince, and seeing no thorn magically turn away, I moved back, regained the main trail, and then took that branch. I wove past walls of grass and bush until the path moved onto a land spit gesturing out into the marsh. In the distance was a major road I had taken thousands of times. I had seen this marsh from the other side, without realizing that it was possible to move this easily into it. The GPS readings suggested that the hiding place was a little depression to the left, with the cache probably behind a tree. Then I heard a loud fluttering; off to the right were two wild ducks beating their wings, lifting themselves off from the wetlands and taking to the air. Then there was a deep rasping, and a large dark bird with a neck like a swan's glided in for a rest. It was a great blue heron, a bird I have never seen live. Sure, I found the cache hidden behind a small clump of trees, but the experience was the real magic.