An increasingly popular game of grown-up hide-and-seek is leading adventurers around the country on high-tech hunts for hidden treasures.
  
It's called geocaching, and all you need to play is a portable Global Positioning System, access to the Internet and a sense of adventure.
  
Players enter co-ordinates in longitude and latitude from a Web site into their GPS, and the hunt is on. Geocachers follow the navigation signal and a list of clues that take them through cemeteries, caves, forests and even historic homesteads.
  
At the end of hunt is a treasure - some kind of trinket or toy - stashed in an airtight container, although sometimes there is only a logbook for players to sign.
  
Sound easy? Sometimes it is - and sometimes it's maddeningly frustrating, said Nicki Sage, 49, of Springfield.
  
In geocaching circles she is know as "Jeepers2," a tribute to the two vehicles that have transported her to myriad sites where she has found over 600 caches and placed 17 more for others to find.
  
"People have asked me if I have found a million dollars. I haven't, but that's not the point," Sage said. "The hunt is the adventure of it."
  
The hunt can include occasional brushes with wildlife, difficult terrain, and hours of work.
  
"I nearly slid off a 45m cliff one time," Sage recalled. "I was on my butt and couldn't find anything to grab onto."
  
Her dog, Memphis, stopped her slide within a few metres of the edge.
  
Sage got involved in the game in July 2001 after seeing a TV segment about it. The game has since taken her all over Missouri and to more than a dozen other American states and Canada.
  
Among her favourite places is Bonniebrook, the former home of Kewpie doll creator Rose O'Neill, 16km north of Branson, Missouri.
  
Sage was so fascinated by O'Neill that she hid a cache in an ammunition container - a favourite because it is airtight - on the property, with the caretaker's permission.
  
The GPS information takes players to within 15m of where the cache is hidden. Players then must use a decryption key to unravel a string of numbers and letters that leads to the container.
  
"I've had so many people tell me that they never would have found Bonniebrook had it not been for geocaching," she said.
  
Geocaching took off in May 2000 after the government removed restrictions on GPS, originally developed for military navigation, said Jeremy Irish, a partner in a Seattle-based company, Groundspeak, that created a leading Web site for geocachers.
  
The change cleared the way for commercial receivers that sell for about $US100 ($NZ140) and are accurate to within 6m, Irish said.
  
Satellites use radio frequencies to broadcast their own positions, and the GPS unit takes that information to figure out where you are, he said
  
"You don't have to wear an aluminium foil beanie to make sure your brain waves are not being scanned," Irish jokes.
  
Irish estimates more than a half-million people regularly play the adventure game in more than 200 countries - from Afghanistan to Zimbabwe. He believes geocaching is growing because the rules are simple: Take something from the cache, leave something in the cache and write in the logbook.
  
Early on, geocaching was more of a rural activity, but "as it's grown in popularity and participants, finding a cache in the city has become much more common," said Nate Irish, Jeremy's brother.
  
Urban caches tend to be hidden in tiny containers, like film canisters, and are frequently tucked away in "little pockets of nature among all the hustle and bustle" of cities, he added.
  
Linda and Bob Shaffer of Mountain Home, Arkansas, have found 378 caches and placed 21 since they started playing the game in 2002.
  
"After you finish the first one, you're hooked," Linda Shaffer said.
  
They use their weekends to go geocaching in surrounding states because they have found all the local caches.
  
Along the way, the Shaffers have had a few "misadventures" - such as the time they were approached by a police officer while searching for a cache under a bridge near Memphis, Tennessee.
  
The country was under a terror alert at the time, and even though the Shaffers explained what they were doing, but the sceptical officer wouldn't budge until they had found their treasure.
  
"That's one of the times when we didn't want to come up without a cache," she said.
  
The Missouri Department of Natural Resources encourages geocaching and estimates there are more than 100 caches hidden in its 83 parks and sites.
  
But a permit application must be approved before a cache can be placed on state property to ensure it doesn't harm cultural or natural resources, agency spokeswoman Sue Holst said.
  
"We see geocaching as an important new recreational opportunity," she said. "We want to provide that opportunity to our visitors, but we also want to make sure that no one is placed in danger."  

Geocachers also pride themselves on giving back by picking up trash while they're out, Sage said.
  
"Being respectful of places where we go is an important part of geocaching," she said.