They have followed the clues, all 15 of them, carefully. They have little doubt that their quarry lies inside the log. Even so, as 12-year-old Rhett pulls the Belle Isle letterbox from its hiding place, Amy utters a gasp: “Oh my God.”

“Whoa,” the Mer-man murmurs.

“Look at it,” Nature Barbie says.

It is an almost mystical instant, the sort of shared revelation at the heart of an old English pastime, “letterboxing,” that combines the scavenger hunt, visual art and orienteering, and now promises to become an outdoors craze throughout the United States.

Thousands of waterproof boxes are hidden in public places in all 50 states — places like Richmond’s Belle Isle, St. Paul’s Church in downtown Norfolk, and the summit of Virginia’s highest peak.

In each is a small log book and a rubber stamp, the latter usually homemade and designed to evoke the box’s location. The hobby’s faithful carry ink pads, their own logbooks and stamps they carve with their personal marks. Following clues posted on the Web, they hunt down the hidden caches, stamping their own books with the letterbox stamp and the box’s book with their own.

Over time, the letterbox logbooks become records of the intrepid souls who’ve trod through the woods and worked out the clues to find them, and a letterboxer’s book: a catalog of his travels, a passport bearing the stamps of places he might otherwise not have ventured.

Letterboxing is said to have started at Southwest England’s Dartmoor National Park in 1854. Over the decades that followed, boxes were hidden by the hundreds in the park, and a small army of enthusiasts sprang up to find them.

It didn’t cross the Atlantic in any large-scale sense for 144 years — not until April 1998, when Smithsonian magazine ran a feature on Dartmoor’s latter-day letterboxers. Within weeks of the story’s appearance the hobby had gained a foothold in a half-dozen, far-flung American towns.That April 26, a box appeared atop Max Patch on the Appalachian Trail near Hot Springs, N.C.,.

Max Patch is a majestic, windswept place an hour’s drive from Asheville, and so became a model for the settings that followed it: Hiding places tend to be difficult to reach, points of great beauty, or both. They’re almost always worth visiting.

Within a year, letterboxes were drawing people off the beaten path to 90 more such places. At last count, the number of boxes with Web-posted clues had grown to 2,312. Ninety-three clues lead to sites in Virginia.

Those clues can make the hunt a mental, as well as physical, exercise.

Some are straightforward directions. Some require the use of a compass. Some, like the Russ Clan’s clues to the St. Paul’s Cannonball letterbox, are sheer poetry:

Find sweet John who was only seven,

he’s next to Fredrick, they are both in heaven.

Next to the two, grows an unusual tree

and there inside is the gift from me.

Likewise, the hand-carved stamps stashed inside the boxes range from florid to stick-figure. The Belle Isle stamp discovered by Rhett and company was a Confederate battle flag. The St. Paul’s box is represented by a headstone, a church’s rose window and the lodged cannonball. The Ovals of Cassini stamp, hidden in the woods at First Landing/Seashore State Park, is a minimalist combination of arcs and lines.

Many letterboxers jot observations about locale, weather or mood in a box’s logbook, but most remain anonymous, adopting a nom de plume or letting their stamps serve as their signatures.

So they devote great care to designing and carving the stamps that will speak for them. Itty Bitty Kitty, a 43-year-old Norfolk corporate vice president, marks her visits with the hand-carved image of a curled cat. Nature Barbie, a Virginia Beach 8-year-old, is represented by a smirking platypus. Rhett leaves behind a wriggling diamondback.

With a little practice, carving even a complicated design isn’t hard. And among the hobby’s many attractions — along with its mystery, the rush of discovery, nature’s sounds and smells — is that it’s cheap. To get started, a letterboxer needs only a notebook, ink pad, and the tools and rubber-carving medium needed to make his or her stamp — a one-time outlay of less than $15.

Then, one need only log on to the Letterboxing North America Web site, the country’s readiest source of clues, at www.letterboxing.org. Clues are organized by states and regions within each, making it easy for families planning auto trips to incorporate letterboxing into their travels.

“It seemed like an adventurous thing to do with the kids,” says Itty Bitty Kitty, who learned of the hobby from friends. “When I went on the Web site, I was excited by the fact that enough people thought it was cool that there were a lot of boxes hidden locally.”

On Belle Isle, the foursome ink their stamps and press them into the pages of the letterbox log — the snake, the platypus, Amy’s steaming cup of coffee, the Mer-man’s primitive sea creature — they take turns stamping the flag into their own books.

Then they carefully repack the letterbox’s contents into Baggies and seal them inside the plastic storage container. Rhett eases the box back into the log. Nature Barbie replaces the rock.

They leave no hint of treasure.

But carry away proof that they’d found it. S