Page eight of the Enterprise Tropical brochure describes the Caribbean island destination of San Andres as 'a tranquil paradise'. But British holidaymakers will have to study the brochure closely to find any mention of the Colombian connection. There it is, in the small print on page nine: 'Currency: Colombian pesos.' San Andres is, indeed, part of the wonderfully wayward nation of Colombia. One thing it is not is 'a tranquil paradise'.

'San Andres is an international duty-free zone, so goods from all over the world are available at reasonable prices.' Here, the brochure gets it right. When the first British tourists step off their charter flight at the island's entertainingly abysmal airport, they will encounter a wall of fridges. Few people visit San Andres to enjoy the scenery. Thousands of mainland Colombians fly in to buy as many microwaves, video recorders and fridges as they can legally and physically carry home. Consequently, the airport looks more like a discount warehouse than an international gateway. You might well speculate about the brochure copywriter who dreamt up the phrase 'a magic place that time has forgotten'.

The 20th century has, by now, remembered San Andres and its smaller sister, Providencia - the two gems in Colombia's rather tarnished crown - 500 miles from the rest of Colombia, but only 150 from Nicaragua (which has a longstanding claim to them). They constitute the alternative Caribbean. The beaches are fringed with the standard-issue palms, and you can do the usual Caribbean things - but at a fraction of the cost.

The Enterprise lead-in price of pounds 585 for a fortnight includes everything: accommodation, meals, drinks, watersports, even cigarettes. No matter whether (or what) you smoke, your stay will be invigorated by the high-spirited verve of Colombia, a country pervaded by a ramshackle air and a faint hint of impending catastrophe.

When most people contemplate Colombia, they probably think of a cocaine manufacturing plant run by scarred hoodlums. But this is as way off the mark as many of the Enterprise descriptions. Visitors quickly develop a fondness for the quirkiness that accompanies almost any activity in Colombia. The national eccentricities seem concentrated in San Andres. The island's capital is one big supermarket, but with a twist. At the El Hogar hardware shop, you can get a Guatemalan visa along with duty-free glassware; the proprietor doubles as Guatemala's consul. For San Andres is more than an attractive tax loophole; it is also a staging post for travellers through Latin America. Until a road is cut through the Darien Gap, the only way to bypass the jungle at the neck of the Americas is to fly. The cheapest way is to get a domestic flight from mainland Colombia to San Andres, then an onward hop to Central America.

So San Andres is full of backpackers. And, because they need visas, it is also full of consulates. Numerous Latin American missions are concealed in gift shops; the Honduran consul runs a cosmetics store called Lady Vanity, which consequently sports an incongruous queue of bearded backpackers. So much for being 'totally undiscovered and unexplored' (page four of The Caribbean According to Enterprise).

Travellers who merely pause as long as the diplomatic bureaucracy requires are missing a remarkable island. San Andres measures eight miles (13km) from tip to toe, and is rarely broader than two miles. On to this modest stage are crammed a full-size international airport, 30,000 residents, a selection of swamps and lakes, and an implausibly large number of refrigerator showrooms. Outside the capital, concrete gives way to clapperboard and the island becomes highly attractive.

The north of the island is dominated by El Cliff, a great slab of rock towering over the capital. Dangling from its skirts is a collection of sad little shacks. This poor barrio reminds you that San Andres is part of a troubled South American country, not a standard tropical paradise. Halfway down is the Big Pond (place names are uncomplicated), where a fearsome collection of crocodiles resides. Completing the onshore attractions is the Blow Hole, a crevasse on the shore at the southern tip of the island. It marks the end of a labyrinth of underwater tunnels; in anything stronger than a gentle swell, water gushes from the ground as impressively as any genuine geyser. This being Colombia, someone has built a beer shed around the crevasse. Spectators swig Aguila beer while the Blow Hole obliges with periodic drenchings to temper the effects of the sun.

A better way to cool off is to plunge into the Caribbean. A coral reef protects the island's only real beach, and provides a visual feast even for those equipped only with a snorkel and mask. Divers regard the reef as superb. Looking back to the island, San Andres is luxuriantly draped in coconut palms. Only an occasional high-rise electrical store blights the view.

The people constitute the next surprise. Colombia is a solidly Hispanic nation, yet most residents of San Andres and Providencia are English- speaking - descendants of the slaves brought in from elsewhere in the Caribbean. They call their home territories St Andrew's and Old Providence. The Spanish first encountered the islands in 1527, but abandoned them because of the absence of precious metals. After a brief occupation by the Dutch, a band of British pirates took over with the connivance of the Crown. San Andres made a perfect base for raids against Spanish galleons carrying treasure from Panama. Captain Henry Morgan - as in the rum - attacked anything that moved eastwards, on the unerringly accurate basis that it was sure to contain cargo. His plunder is said still to be hidden in an underwater cave.

As the fruits of piracy diminished, efforts were made to develop the agriculture of the islands. Some time in the 19th century, when the attentions of European colonialists were concentrated elsewhere, Colombia made a grab for ownership. Despite the political upheavals since then, the South American nation shows no sign of relaxing its grip on these two distant treasures.

The collision between Caribbean tradition and Colombian unpredictability is evident in the food. Beachside restaurants on most Caribbean islands pride themselves on elaborate, mouth-watering menus and exotic cocktails. But this is Colombia. Perched on the seashore at the top end of the airport runway is a rambling shack called the Fisherman's Place. Here you can dine on anything as long as it is fish and beer. The menu is breathtakingly simple: red snapper, sold by the kilo and cooked in front of you, and Aguila beer.

Just along the promenade, but at the opposite culinary extreme, La Fonda Antioqueno is the island's classiest restaurant. Arguably the best cooking in Spanish-speaking America is Colombian, and Antioqueno cuisine from the city of Medelln is the nation's finest. On San Andres you can taste the amazing things that can be made from meat, rice and tropical vegetables. And then you can fly to Providencia.

The pilot of the neurotic-looking nine-seater insists on wearing sunglasses throughout the 20-minute flight to Providencia, even if the landing has to be made through thick cloud into a rainstorm. He has a monopoly on the route from San Andres, and charges pounds 10 to scare his passengers witless on the way to the northernmost fragment of South America.

Providencia's airport terminal consists of a wooden hut, unsullied by fridges. The reservations system for flights back to San Andres (the only destination) comprises a school exercise book. Oversize dollops of warm Caribbean rain - more like grenades than drops - fell relentlessly throughout my stay, but excessive precipitation is necessary to sustain Providencia's triumphant greenery.

A gentle 'Welcome to Providence' greeted the traumatised and instantly soggy passengers trudging from the aircraft. It issued from the man who runs the island's entire public transport network: an old school bus, which circles the 11 damp miles around the island's only road. He does not charge fares: as long as you share this entrancing island, the logic seems to go, why should you be expected to pay for something so commonplace as a bus ride?

Providencia is tear-shaped, and unfurls from the 1,200ft (400m) peak in the middle. The four modest centres of population are placed neatly at the compass points. Due north is the capital, Santa Isabel, whose haphazard wooden houses and chapels merge casually with the jungle. The airport, a patch of bumpy asphalt like a long, thin car park, clings to the eastern edge of the island. A few Colombian servicemen, grateful for this prime posting, occupy a naval base to the south. Only a handful of leisure travellers make it to Providencia, and their modest tourist enclave occupies the western coast around the hamlet of Lazy Hill.

The chores are undemanding: to select which tropical fruits to devour for breakfast; to ponder if that blurred scrawl of ballpoint in the airport reservations book bears any resemblance to your name; and to plan excuses to explain your failure to leave. Regrettably, my number came up and I flew south, bound for Cali, the cocaine capital of southern Colombia. Compared with the eccentricities of the islands, Cali seemed positively normal - except for the number of fridges lumbering around the baggage carousel.

Simon Calder is co-author, with Emily Hatchwell, of the Travellers' Survival Kit: South America (Vacation Work, pounds 12.95).

Getting there: I bought a discount ticket on the Colombian airline Avianca (071-408 1889) from London to Bogota. This costs pounds 449 in low season, but pounds 540 in July, August and December. I combined it with a Colombian airpass valid for eight flights, price pounds 230 low season, pounds 250 high.

Package tours: Enterprise (061 745 7000) begins package holidays to San Andres in May. The cheapest fortnight costs pounds 585 from Gatwick ( pounds 10 more from Manchester), all- inclusive. In high season (July/August) the price is pounds 885.

Accommodation: Easy to find in San Andres. I stayed in the Hotel Restrepo at pounds 3 per person per night - but the airport runway is only 50 metres away. Pay two or three times as much, and you get a bathroom as well as peace and quiet. On Providencia, lodgings are scarcer and prices higher. The Cabanas Aguadulce has comfortable rooms and good breakfasts for pounds 20 a night.

Further information: Colombian Consulate-General, 140 Park Lane, London W1Y 3BF (071-495 4233).

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