I am, however, supposed to be at N 37° 54.637 W 122° 37.52. It would appear from your vantage point above me that I am really almost where I need to be, mere decimal points away -- a distance equal, as the crow flies, to just 100 or so yards. Unfortunately for me, down here on the solid ground, those few decimal points lead to the bottom of an extremely steep hill, down to which there is no apparent path.

And in the game I'm playing, those decimal points really, really matter.

On May 1 of this year, President Clinton signed a bill that brought an end to the practice of Selective Availability for public GPS (or Global Positioning System), a military-run location-finding system that uses satellites to precisely map that grid. For years, GPS users were resigned to the fact that their nifty hand-held GPS devices weren't particularly accurate: The U.S. military regularly degraded the signal in order to prevent data from getting into enemy hands. But in May, the military finally decided to give 100 percent accurate positioning to the public, and suddenly GPS systems that had been accurate only to within 300 feet could place your exact position to within 30 feet.

On that day, a game called "geocaching" was born. And that's why I'm here, standing on this mountain overlooking the sea, listening to the wind in the tall grasses and watching the hawks circle overhead and the silvery waves break far down below me, wondering just how I'm going to make it down this hill without breaking an ankle. If I can do it, treasure awaits.

The concept of geocaching is really quite simple: Someone hides a "stash" -- usually a large Tupperware container filled with assorted goodies -- in an interesting, out-of-the-way place, and records the exact coordinates with a GPS device. Those coordinates, along with a few helpful hints, are posted to the geocaching Web site. The stash seekers then use their GPS systems to find the treasure. Each person who locates the stash adds an entry to the included log book, takes one of those goodies, replaces it with one of their own, and then re-hides the container.

The fact that this is harder than it sounds is one I am learning as I gaze over the precipice. The directions for the stash that I am seeking -- the Firestone Stash, it's called -- were posted in October with a note that said, "Today I placed the first stash in the San Francisco Bay Area. I have named it 'Firestone' for reasons that will become evident to anyone that finds the site. Look in the trunk for the goods."

The map on the Web site pinpointed the stash somewhere in the middle of Mount Tamalpais State Park and so, with a GPS hand-held device in my lap, I had driven slowly back and forth until I found what seemed to be the parking space with the closest GPS coordinates.

What GPS devices can't tell you is that geocaching is as much about the route as the location. And so I begin half-sliding, half-climbing my way down the hill, all the while keeping an eye out for what I'm guessing, based on the "Firestone" and "trunk" clues, will be an abandoned car.

And I'm paying such close attention to the coordinates on my GPS receiver, watching as each step ticks me one digit closer to my destination (and marking, as well, the time and distance that I've traveled), that I don't even see it until I'm almost upon it: the rusted carcass of an ancient car, upside down and resting against the tree, legacy of a long-ago accident. Decades ago, at least, someone probably died here when they took a curve too fast and hurtled down the hill. Eureka.

The Free Software Project
Read Andrew Leonard's book-in-progress on Linux and open source -- and post your comments.