A telephone ringing in an empty call box; a classified advert for a missing cube; a Web site of bloggers who reply to your messages from a city that does not exist. This is the strange world of alternative reality gaming -- usually an innocuous way of wasting time on the Internet, but now offering the very real prize of ?100,000 (US$178,380).
Tens of thousands of people across the world are taking part in what could be the biggest treasure hunt in history: A part-real, part-virtual game which will lead one of them to an object buried somewhere on the planet.
To find the Receda Cube and claim the cash reward, players of Perplex City have to spot clues in the real world, collect puzzle cards and enter the solutions on a Web site, interact with fictional characters and other players, then use powers of deduction worthy of Sherlock Holmes. The competition is intense: Everyone in the world can take part.
In the era of The Da Vinci Code and Sudoku, it is an idea whose time has come, but the game's creator, Michael Smith, was inspired by a book published in 1979. Masquerade, by the artist Kit Williams -- which contained clues to the whereabouts of a jewelled golden hare he had buried at Ampthill in Bedfordshire -- sold more than a million copies and sparked a frenzy of nationwide digging. One German sold his home and moved to Britain to join in, while Fred Hancock drove 180,000km, digging scores of holes hundreds of kilometers from the treasure.
The people of Tewkesbury, Gloucestershire, had gardens dug up and Haresfield Beacon, a National Trust site near Williams' Gloucestershire home, was carved up by diggers. Williams had to issue a statement saying the treasure was not buried on private property.
The hare was found after nearly three years by a man linked with one of Williams's ex-girlfriends, prompting the author to say he felt "conned."
Perplex City has already gained nearly 40,000 registered players since its launch in April. Stunts so far have included a series of cryptic messages, asking for help in locating "The Cube," in the Guardian, Times, Los Angeles Times and the South China Morning Post.
One character turned up at the Beatles-famed zebra crossing on Abbey Road in north London to hand over clues to around 20 players who had worked out the time and place of the rendezvous.
Smith, who has spent two years constructing the virtual city, which even has its own newspaper, said: "We're trying to create a modern version of Masquerade -- a game that spills off the Internet."
"We're hoping millions of people will get involved and turn this into a massive international experience. When the cube is finally found, we'll start another game," he said.