Hikers, you've got mail!

The box containing the secret message is hidden in the woods at the base of a white pine tree. Jason Wilmot brushes away the dry leaves, revealing the plastic container. His girlfriend, Amy Nelson, opens the lid.

Inside is a tiny notebook and a communique written by someone who came this way two weeks before: ''Very nice walk on a beautiful spring day. Thanks. Bubba's Family."

Welcome to letterboxing, a little-known treasure hunt hobby in which people hide small boxes in the woods and leave directions or clues on the Internet so that other people can hunt for them.

If letterboxers find a box, as ''Bubba's Family" had found Wilmot's and Nelson's, they leave behind written messages and, using artistically carved stamps, their own personal emblems. They also use a stamp provided in the box to mark the journals they carry, giving them a record of another successful quest.

The boxes are hidden across the state, including the western suburbs, and enthusiasts say interest is growing in a hobby that allows them to enjoy nature and express their creativity.

An organization called Letterboxing North America provides directions to some 15,000 letterboxes, including nearly 900 in Massachusetts, at www.letterboxing.org.

''There's been a 100 percent increase in boxes in Eastern Massachusetts in the last 12 months," said John Chapman, 49, of Andover, one of the organization's webmasters, who himself has placed letterboxes in Wellesley, Needham, and Newton as well as in the Andover area.

Letterboxing.org lists individual letterboxes by state and county. It explains how letterboxing works, provides links to related websites, and offers tips on how to carve your personal stamp and how to write clues to letterboxes you hide.

Letterboxers who use the site also adopt sometimes fanciful ''trail names," which allow them to protect their privacy.

Letterboxing traces its roots back 150 years. The legend, according to Letterboxing North America, is that in 1854 ''a gentleman simply left his calling card in a bottle by a remote pool on the moors of Dartmoor, in England." The calling card evolved into a unique signature stamp. Today there are thousands of letterboxes hidden in what is now Dartmoor National Park.

Enthusiasts trace the beginning of letterboxing in America to the publication of an article in Smithsonian magazine in 1998 about the hobby in England, but it started slowly in the United States and remains somewhat obscure.

''It's fairly small still, which in my mind is good," said Chapman.

The hobbyists feel the obscurity keeps their pastime special and helps to protect the environment, since most letterboxes are placed in forests, parks, and conservation areas. (To keep searchers from trampling the environment, boxes are usually hidden close to public trails.)

Government agencies have reacted to letterboxing in different ways. The state of Connecticut plants its own boxes to encourage people to take advantage of its 32 state forests. National Park Service lands, on the other hand, are generally off limits to letterboxing. The Massachusetts Department of Conservation and Recreation is developing guidelines for placing boxes on state lands.

Some letterboxing clues can require the use of a compass or a GPS (global positioning system) device, which lends it a resemblance to orienteering, a hobby in which people use a map and compass to navigate through the woods, or geocaching, a hobby in which people use GPS devices to find small boxes with inexpensive items in them.

Cynthia Zerquera, 46, of Milford, whose trail name is Warrior Woman, got involved in letterboxing in 2000 in Rhode Island, where she also has a home. After discovering that there were few, if any, letterboxes in the area, she went on what she calls a ''planting frenzy" in 2001, hiding boxes in Milford, Upton, Hopkinton, and Westborough, among other towns. She said she has planted nearly 50 letterboxes, including one at Dartmoor National Park, and has found about 500.

Originally, letterboxing typically involved a long hike leading to a single box ''in a breathtaking location" or a mystery to solve with difficult clues, she said. Today, there are often shorter walks and simpler clues -- and sometimes a series of closely placed boxes. That seems to reflect the pattern of more people making letterboxing a fun family activity, with clues or directions that children can follow.

''It's a great thing for families to do together," said Pam Musk, director of Stony Brook Nature Center in Norfolk. There are two letterboxes at the busy Massachusetts Audubon Society facility, according to Musk. One had to be moved slightly to prevent habitat damage, but the clues still work, she said.

The nature center is neither prohibiting letterboxing nor encouraging the placement of more boxes. ''Stony Brook is 245 acres, and most of that is underwater. Two is probably a good number for this site," said Musk.

Zerquera said all her boxes have themes. For instance, a series of seven in Westerly, R.I., is devoted to one of her favorite movies, ''Blade Runner." Her prologue to the clues begins, ''Deckard landed his spinner at the pad at Westerly Airport. He took a land vehicle and headed out onto Route 1 North." The directions follow from there, with frequent allusions to the movie and its characters.

Chapman, the Andover letterboxer, took his trail name of Choi from a character in the movie ''The Matrix." He said he has created a series of about 10 boxes ''celebrating aboriginal rock art from around the world." Chapman said he is one-eighth Cherokee and most of the stamps relate to Native American art.

Some letterbox clues tell poignant personal stories. The directions to the Father of the Bride Letterbox in Edgell Grove Cemetery in Framingham include requests to ''pick up a small rock and put it in your pocket" and to fill a container with water from a spigot in the cemetery.

The letterbox searcher eventually reaches the grave of Eugene Maloney. ''Take the small stone out of your pocket, say a prayer or whisper a kind thought into the stone, and leave the stone on the headstone for Gene. If you could water the flowers, too, we'd surely appreciate it," the directions continue before adding the final clues that lead to a letterbox hidden nearby.

This letterbox belongs to Carol Ames, who grew up in Framingham and now lives in Worcester with her husband, Ken. She placed the box in the cemetery on their wedding day, May 29, 2004. Her father died in 1999, at age 69.

''My dad, Gene, loved to walk in the woods, including this cemetery," Ames, 36, explains in her directions to the letterbox. ''He walked every day, to loosen bad neck and back muscles and soothe a busy mind. The walk he couldn't take, however, was with me on my wedding day. This is for him."

Ames said in an interview that a number of people ''have written very nice notes in the log book" and also apparently watered the flowers, because they survived all through last summer.

As the pastime has grown, letterboxing has taken on the attributes of other hobbies.

There are gatherings of devotees where personal stamps are exchanged and stories shared. Some letterboxers become competitive, trying to reach various milestones of boxes placed and found. A few have a goal of placing a letterbox in all 50 states.

A number of variations have also sprouted, including virtual letterboxes that exist only on the Internet, and ''hitchhikers," items within letterboxes that the finder takes and then leaves in another box. ''There's endless creativity, endless variety," said Zerquera.

Wilmot, 29, and Nelson, 28, of North Attleborough, who both grew up in Wrentham, decided to check on their ''Wetland Wonders!" letterbox recently at the Crocker Pond conservation land to make sure it was intact. The box is one of about 30 the two have planted in several area towns over the past couple of years.

''I'm curious to see who found it," Nelson said as she prepared to open the box.

The two logbooks inside contained the stamps of about three dozen other letterboxers. The most recent visit was by ''Bubba's Family." The stamp created by Nelson and Wilmot, who go by ''Amy & Jay" on the Web, was made from a small block eraser. It has the raised image of a frog on one side and a dragonfly on the other.

Nelson got some turquoise ink on her hand from the stamp pad as she displayed the items in the box. ''That's the sign of a letterboxer," she said.

A nature educator for the Department of Conservation and Recreation, she said she learned of the hobby about two years ago and was instantly taken by it because she has had a longtime interest in the outdoors.

Wilmot, a machinist, said the hobby gives him another reason to scout out trails where he can pursue another interest, mountain biking.

They also like that they can do letterboxing together. The clues to a box they once placed on the top of Sweatt Hill in Wrentham ended, ''P.S., Jay loves Amy and Amy loves Jay."

Ned Bristol can be reached at nedbristol@msn.com.