Hey Mr Postman: There used to be a letterbox on every corner, but nowadays you need a map and compass to find one. Roberta Mock takes her bearings

Imy and I are two city gals game for a bit of mystery and outdoor adventure. So when Annie offered to take us to Dartmoor to search for letterboxes, we laced up our DMs and bought extra supplies of rolling tobacco in eager anticipation of the journey. Experienced Annie was far better prepared. In the back of her Fiesta we discovered Ordnance Survey maps, water supplies, an orange plastic survival bag, and the latest edition of the Catalogue of Dartmoor Letterboxes.

Annie had spent 45 minutes planning our itinerary with military precision. We drove through outrageously twee Widecombe-on-the-Moor and parked the car on a remote verge. Annie pointed at the looming tor we were about to climb and whispered gleefully, 'There's bound to be dozens up there.' Imy and I shuddered. We didn't realise that there was climbing involved in this activity. Nor did we see any red pillarboxes.

It was here we learnt lesson number one: letterboxes are not real letterboxes. They are steel boxes or mouse-proof plastic canisters hidden beneath heather and scrub. You can only find them using clues supplied in the letterboxing catalogue, and you can only decipher the clues if you know how to read a map and use a compass. And there's an added twist: you are only entitled to buy a Catalogue after you've found 100 letterboxes, either with the aid of a guide like Annie or sheer bloodyminded persistence.

More than 10,000 people belong to the 100 Club. But it is not a real club; it has no constitution and the man who doles out badges and membership cards takes it on trust that prospective members have indeed stumbled across 100 letterboxes. Once a member, a 100 Clubber is allowed to hide and maintain his or her own letterbox with its own name, which means there are well over 10,000 boxes on Dartmoor.

Godfrey Swinscow is the sprightly septuagenarian who is the non-leader of this non-organisation. 'People like to belong to clubs,' he says. 'Letterboxing is a complete leveller. You can't tell if somebody is a managing director or an out-and-out rogue when they're on the moor.'

Letterboxing began in 1854 when, in a typical act of Victorian whimsy, a Devonian guide named James Perrot placed a bottle in which walkers could leave their cards under a cairn at Cranmore Pool. This was eventually replaced by a more permanent receptacle, and copy-cat 'letterboxes' were established. When the 100 Club was founded by Swinscow in 1979, there were still only 101 letterboxes.

Given the sheer quantity of the things now, Annie, Imy and I felt confident of a find. We puffed uphill and looked through the catalogue for likely candidates. 'Sweet Harmony' proved frustratingly elusive. After 40 minutes, we gave up and began our search for 'Tramp's Box' on the south-east side. After scrambling through spider-web-festooned caves with no success, we moved on to 'Pooky's Box'. 'Pooky has escaped Garfield's clutches,' read the clue. Ours too. I began to suspect the whole set-up was an elaborate hoax.

In sheer desperation we set our sights on the 'Honeypot Tor' box. We travelled from a small quarry to a huge flat rock and then took 15 paces in the prescribed direction. We spread out and combed the slab. 'Stop the clock,' yelled Imy, hopping excitedly.

She had found it. Our first letterbox. We prised it open and added the Independent to the names in the Honeypot Tor visitors' book. The box, like all others, contained a rubber stamp, and we gleefully inked its emblem into our notebooks as proof of our success. Invigorated, we found another five letterboxes that day, but none was as sweet as that first discovery on Honey Pot tor.

Book: 'Dartmoor Letterboxes' by Anne Swinscow ( pounds 3.95 from Cross Farm, Diptford, Totnes, Devon TQ9 7NU). For information send an sae to Godfrey Swinscow at Cross Farm as above

(Photograph omitted)