If it surprises you that anyone would care enough to take Gunn up on her challenge, then you've probably never heard of geocaching, a high-tech treasure hunt played with handheld versions of the same GPS receivers that have guided missiles with such success in the war in Iraq. The sport, which started in a small way three years ago when the U.S. government opened up its network of 24 navigational satellites to civilian access, has lately taken off. The site on which Gunn posted the location of her marbles, geocaching.com, boasts more than 100,000 members and 50,000 caches. Some 1,000 new caches are listed each week, hidden everywhere from Easter Island to Estonia.
Now Garmin International Inc., a leading GPS-device maker, is tapping into the craze by incorporating location-based games into its handheld units. For example, its new Geko 201 ($150) invites users to traverse a virtual maze to capture a series of imaginary flags. To play, stand in any field or parking lot (you need at least 360 sq. ft. to maneuver) and look down at the screen to see where the nearest flag is located. Then walk or run toward it. An onscreen arrow updates your location and tells you when you've reached the flag. You can play alone or with friends, but make sure you look up every now and then, so you don't run into a tree as I did while hunting for flags.