May 22, 2007--the day we've been waiting for--has dawned glorious. But it's not the sunshine that fills us with joy. It's the half-foot minus tide, which means the upper edges of Monterey Bay will be a little less sloshy than usual. It is a perfect morning for a treasure hunt.

I check to make sure that my knapsack is ready, with GPSr (global positioning system receiver), plenty of spare AA batteries, and a Ziploc goody bag full of knickknacks. A camera too--you never know when the hunt will take you somewhere unexpectedly beautiful or interesting. And last but not least, a sheaf of clues.

Our tick-list today features two prominent spots that we've been to but have not been able to check out because they are usually under water, or at least wave-swept. We had stored both in the GPSr as latitude-longitude readings ("waypoints"), and since then have been waiting for a minus tide. Today they will be within our reach. One of them, at the tip of the Monterey Peninsula in Pacific Grove, is usually an island; the second, a crumbling 20-foot-high structure left over from the Cannery Row heyday--perhaps a former pump station--is sometimes at the water's edge but more often sloshed by waves. Both promise special rewards--assuming our searches prove successful.

What is it we seek? Tupperware and old ammo cans, or, on the "micro" end of the spectrum, test tubes, film cans, and Altoids tins. The containers vary, as does the treasure stashed inside (think shiny colored objects or at the very least a minuscule logbook), but we can always be assured of finding something. If, that is, our skills of navigation and observation are up to the task.

The pastime we've become addicted to is called geocaching: geo- as in earth (it's a worldwide hobby, with more than 410,000 caches in 222 countries worldwide, and it also takes place out in the world--a definite plus); and -caching, as in squirreling something away; and it takes you out into the world on a glorified scavenger hunt among tens of thousands of strangers.

Geocaching is a game made possible by military surveillance, including satellite-enhanced position finding--something that was available to civilians only in an intentionally degraded form until May 2, 2000. On that day, a mythical "great blue switch" was flipped, and in an instant the accuracy of GPS technology improved tenfold as 24 satellites processed new orders and allowed backpackers, boaters, scientists, and other citizen users to join the military in gaining pinpoint positioning accuracy.

The move itself was not entirely unexpected. The White House had earlier declared its intention to cease data degradation eventually--say by 2006. For the "switch" to be pulled six years early--and announced by President Bill Clinton's press secretary just one day before the fact--came as a total surprise. Internet technogeek newsgroup sites immediately sizzled with ideas on how to take advantage of the new capability.

One GPS enthusiast, David Ulmer of Beaver Creek, Oregon, decided to put it to an immediate test. On May 3, 2000, he posted a notice on a GPS users' bulletin board, sci.geo.satellite-nav, saying he had stashed a bucket in the woods and its contents were a logbook and pencil, as well as such goodies as videos, software, and a slingshot. He gave these coordinates of his cache, read from his now more powerful GPS device: N45°17.460 W122°24.800. "Take some stuff, leave some stuff," he invited anyone interested. And so the Great American GPS Stash Hunt was born, redubbed "geocaching" within a few months.

As one wag noted on a keychain we found early on, "We use multimillion-dollar military satellite systems to find Tupperware in the woods. What do you do?

Liberating Minnie
We decide to hit the cannery tower first. It's near Monterey's Coast Guard Pier, on a beach where SCUBA diving certification tests are often held.

We learned about this cache, called PDH, from the official geocaching website (www.geocaching.com), which provides difficulty ratings, descriptions, encrypted hints, maps, and visitor logs and photos. The cacher, Touchstone, stashed the hide on December 12, 2005. Out of a possible five, he gave it two stars for difficulty of finding and a stiff four and a half for difficulty of terrain. He then posted a description:

"The cache coordinates will take you to one of the old cannery buildings at the popular and scenic San Carlos Beach. A very busy place on weekends, but don't worry, you should have plenty of coverage to enjoy the site and make your trades in leisure.

"There is a large plaque explaining the cannery operation on the sidewalk above the cache location if you're interested in reading about it."

The cache name comes from a rating system invented by Jim Bridwell during the 1970s for big-wall rockclimbs in Yosemite. His CRS (Casual Rating System) had four levels: NBD (No Big Deal), NTB (Not Too Bad), PDH, and DFU. Since this is a family magazine, I'll leave it as an exercise for the reader to decipher the last two ratings.

David and I are climbers, so we figure PDH can't be too daunting.

Ah, well. This tower is, in point of fact, vertical, as in straight up: 90 degrees. Gravity has a certain edge on vertical walls. And the tower is 20 feet tall and seems to grow taller as we gaze up at it. But yes, it is also crumbling: there's texture; there are holds. And--this is the clincher--there's a cache inside.

David puts on his rock shoes--yellow Boreal Ninjas--then steps onto the wall near the tower's edge, grasping a corner up high with his right hand. He steps down. Looks up. "It's hard on the fingers," he mutters. He steps on again, reaches up, moves his left foot high, then frantically grasps for a piece of sheared-off, down-slanting concrete. It's a solid undercling. But what now? He's halfway up. There's nothing to do but dyno--lunge upward and hope he can grab that metal rebar at the top, which fortunately is smooth and of good length. And so he does. He grabs, pulls himself up, teeters a moment, then peers inside.

"Do you see the cache?" I ask.

"I see a ladder. It's rusty. I'll test its sturdiness." He continues to survey the tower's interior. "There's also a lot of junk in here. A rusty camshaft, for one thing. And plants. But at least it's not wet."

"Yes, but do you see the cache?"

"Not yet. There's a pile of driftwood in the corner. Maybe it's under there."

He drops over the side and disappears. I wait. "Take some pictures!" I yell.

A few minutes later, his head pops up over the top lip. He raises his arm in victory. Clutched in his hand is . . . Minnie Mouse? "Found it," he says. "And I've liberated Minnie."

He twists his back to me, dangles his body from the top of the wall, and drops to the damp sand.

Although I had decided the tower was too risky to climb, I am of course consumed by a need to see what's in there. David's shoulders provide a solid platform, and I quickly climb up, over, down, inspect the cache myself (Minnie was the prize, for sure), and climb back up and out.

It's our 64th cache. And by far the hardest.

Travelbugs and Geocoins
After bagging this find, we head south to search for "Kashta's Rock." The cachers, Otter Girl and Keyholelimpet, warn us, "When you see this point on your GPS it may look as though the point is in the water . . . IT ISN'T IN THE WATER. Rest assured, it's on dry land."

Ah yes, low tide is our friend. We cross some coarse sand--which obviously is under water much of the time--and then climb onto a low island. "Yes it's really out there," the cachers assure in their hint. "Go at low tide; now that you're there [at the posted coordinates], you're standing on it." The GPS switches from tenths of a mile to feet, and the countdown begins: from triple digits--500s, 400s, 300s (ooh, we're getting close), 200s, 100s--to double digits. We start looking around.

Sometimes, for various reasons--tree cover, power lines, not many satellites overhead--accuracy isn't truly "pinpoint." It can be 50 or more feet off. But you learn to use your eyes. Look for a pile of wood that doesn't seem quite natural, or some mussed-up pine needles. Or in the case of Kashta's Rock, a big stack of medium-sized stones clustered under a large boulder. No way would the sea have deposited them like that.

I start pulling stones out. And there, in the back of the alcove, is an olive-green ammo can. I open it, pull the logbook out from its Ziploc waterproofing, and start to inspect the contents of the container. There's a travelbug (a TB): Ole One Eye is a little green fellow waving happily at me. A card attached says that he was "released" in Illinois on July 6, 2006, and his goal is "to travel the earth seeing the sights and collecting as much information as possible before I return to my home planet to make a complete report to the BIGEYE." So far he's been to Arkansas, Indiana, Iowa, Michigan, New Mexico, Oregon, and California. Since we're heading to New York in a few weeks, we'll give him a lift.

Travelbugs, along with geocoins, are "trackable" items that inveterate geocachers have made and assign tracking numbers to; their journeys can then be followed on the geocaching.com website. In my treasure Ziploc, Ole One Eye joins a TB Jeep (green) and a geocoin, created by ShadowAce and Elmosmelmo, that I picked up in Arizona. ("The world is our playground," as they say.) I will drop them off on our trip out east and, I hope, pick up some new traveling items to bring back home. Being a travel addict myself, I can appreciate the urge to keep on moving.

Geocaching is about a lot of things, including: timing, as in tides, or just in mood; using your senses; and second-guessing (that old GPSr may be 10 times more accurate now than in April 2000, but sometimes it's still not very close). It can be about puzzles, too, as some caches involve multiple steps, while others require that you sniff out clues or solve problems to determine the correct waypoint coordinates. Even though cache contents are often silly, there's always that moment when you open the container and look inside: Will there be a travelbug? And it's fun to read the logbooks, which contain comments of people from all over the world.

In addition to being fun, geocaching can be instructive. A cache called "Snowy Plover," for example, along the bike trail in Seaside on the Monterey Peninsula, provides a public service announcement on the little bird's plight along the West Coast, while "Western Sloughs--Global Warming" treats the reader to a 20,000-year history of the area we now know as Elkhorn Slough. Historic buildings, colorful local characters, significant events (were you aware that the last commercial Morse code maritime transmission in the United States was sent from Half Moon Bay on July 14, 1999?)--you never know what you're going to learn about when you log on for another dose of geocaching.

Sweet Little Secrets
The pleasures are deeper, of course, than uncovering bright little objects or even learning new facts. On one hunt recently in the Huachuca Mountains of southern Arizona, I set out on a 14-mile loop trail that took me through various biomes of that beautiful "sky island" ecosystem. I stopped to photograph alligator cypress, agave, and other plants unique to the area. I looked up and watched hawks soar and songbirds flit. Lizards skittered over rocks and smiled at me. Along the way, I ventured off the trail to explore an old mine site (and locate a cache called "5471 Broadway") and again to sit on a peaceful rock outcropping, and finally I stopped at an old ranch that has been preserved as a historical site (Brown Canyon Ranch). As a bonus, at several points where I came upon an unsigned fork in the trail (the only map I had was a sketchy one in my head), the GPSr suggested the correct way to go. It is good for practical navigation as well!

At home, too, thanks to geocaching, we've discovered luscious hiking spots we never imagined existed, the trailheads being no more than unmarked scuffs on the roadside. We've edged out beyond the bluff trail in Pacific Grove onto a jumble of rocks that invited us to sit and watch as cormorants preened, pelicans sailed by, and belly-flopped harbor seals eyed us lazily. At a spot a 10-minute walk from our house, in the Monterey Peninsula Regional Park District's Frog Pond Reserve, a test-tube microcache is hidden in the struts of a bench in a small grove of redwoods. We walk here often, but now we are more likely to stop a bit and sit on the bench. We don't need to look at the cache again. It's just a sweet little secret we enjoy sharing with a bunch of strangers.

Today, we continue to work our way around the peninsula, ending up late in the afternoon at "Kegan's Cache," high on a hill in Seaside. We easily find the microcontainer (it holds a log only), and then pause to look out over Monterey Bay. The sun is blinding as it reflects off the water, silhouetting the distinctive topography of the peninsula. We never would have gotten this view without geocaching.