Strange and inexplicable things happen off the south-west corner of Sardinia. On the short, choppy ferry journey to the offshore island of San Pietro it's as if the ship's compass has suddenly gone haywire and you are being transported to the wrong end of Italy. Appearing out of the mist, numerous houses along the seafront of the island's only town, Carloforte, are washed in pastel shades that belong to somewhere else. But where, exactly?
Stepping ashore, I try to orient myself at a gathering point; a place where people exchange the time of day. Set back a short distance from the seafront, I come upon Piazza Pegli, an open square with a waterless fountain where strutting youths pose with their mobile phones, pretending to ignore a seated, chattering gaggle of older folk. The dialect is unfamiliar and harsh – quite foreign to the south. I soon discover that the traditions, surnames and cuisine of these people do, indeed, originate from afar. In British terms, it's the equivalent of setting off for the Scillies and landing, somewhat dazed and confused, in Aberdeen.
The history books unravel the mystery. Lying next to an important sea-lane, San Pietro was almost deserted until the 1730s, when the King of Sardinia, Carlo Emanuele III, decided it needed a fortified town to keep marauders at bay. The chance to build a new life on a fertile island came just in time to save the remnants of a Genovese community that had settled on the island of Tabarka in the Bay of Tunis, about 170 miles to the south. The inhabitants were regularly falling prey to North African pirates, who sold those they captured into slavery. In the spring of 1738, the survivors sailed for San Pietro, bringing their Genovese ways with them. The Piazza Pegli, where I first encountered island life, is named after a district of Genoa.
If anything, the islanders' emotional connection with northern Italy has intensified down the generations. As recently as 2004, San Pietro became one of the 68 communities belonging to the Province of Genoa, even though politically it's connected to Sardinia. The main statue on the esplanade in Carloforte town betrays the somewhat lukewarm relationship with that big island on the eastern skyline. It was built by a sculptor from Genoa shortly after King Carlo Emanuele's death in 1773, to commemorate his freeing of the settlers from slavery. A male and female slave pay homage at his feet, but the monarch, in true Italian style, is more an object of mirth than reverence. Most of the carelessly constructed statue's right arm broke off long ago. No one bothered to replace it. The king's left arm was broken by an exultant football fan, trying to plant a flag on his head after Italy won the World Cup in 1982. The severed limb was repaired, but the break is clearly visible and its owner looks quaintly ridiculous.
There are strong echoes of San Pietro's Tunisian background, too. A festival every April celebrates the couscous that formed the basis of the settlers' diet. The local wine, Tabarca, is named after their former home, and around the island there are squat, whitewashed houses with a distinctly north African look, containing basement rooms known as cisternas, designed for the collection of precious rainwater for the dry summer months. In some houses the water is still used for washing and cooking.
The life-blood of the island is, and always has been, tuna. Since Phoenician times, red tuna have been taking the same route past San Pietro's north-east coast for their annual spawn, when the meat of these creatures apparently tastes at its best. The tuna run lasts for six frenetic weeks from May. Large numbers are trapped in nets and hauled on to boats for the Mattanza, the annual slaughter. San Pietro would have sunk without it, and the seafront bars are decorated with photographs celebrating the biggest catches. Most of the tuna is exported, but the islanders keep enough for themselves, and pasticcio alla carlofortina – tuna, home-made noodles and tomato and basil sauce – is Carloforte's culinary pride and joy. There's even a four-day tuna festival at the end of May, when chefs come from around the world for a tuna-cooking competition.
The Carlofortini, as the 6,500 islanders are known, regard their remote location as cause for celebration. Rosanna, a guide, shyly sang for me the island's traditional song that translates as "Green, Faraway Island", much as Scottish islanders ("over the sea to Skye") make a virtue of not belonging to the mainland. At summer's end, San Pietro stages a colourful festival of street theatre, dance, puppet shows and stilt-walking, with the tongue-twisting title of dall'isola dell'isola di una penisola, meaning "From the island [San Pietro] of an island [Sardinia] of a peninsula [Italy]".
The greenness of the island is as intense as the deep blue of the sea. The central area is dense with pines, and vegetation grows down the slopes and hillsides almost to the water's edge. I walked along the west coast through clumps of wild rosemary, orchids and succulents – some of them unique to the island. From the cliff-tops, the water was so clear I could see the stones of the sea-bed, 200 metres below. I clambered down winding footpaths to small, crescent-shaped white beaches sprinkled along the coast.
In sweltering August, visitors from Sardinia and the Italian mainland can swell the population five-fold, but at other times of the year the island reverts to rustic tranquillity, when its delights and oddities can be enjoyed at leisure.
One of those oddities was created by Giorgio Mazzucchetti, who was born in Genoa and first came to San Pietro as part of the madding holiday crowd. There he experienced a life-changing moment. Giorgio learned that a miniature species of donkey, the asinello sardo, which had occupied the island since the third millennium BC, was in danger of being wiped out. Large numbers were being run over on the busy roads; many more were being killed for their tender white meat, and 10 years ago their population had dwindled to around 130. On a whim, Giorgio decided to abandon his career as an industrial consultant in Milan, and built a sanctuary on his favourite island.
He bought a house with some rough land in a remote wooded valley near the west coast, built a yard and stables, studied every book he could find about the impossibly cute but notoriously stubborn creatures, and began a breeding programme.
Eight years on, and now in his sixties, Giorgio and his wife Cristina have a thriving herd of 60, with another 15 expected this year. Reared on a diet of hay, cereals and vitamins, the herd has suffered none of the diseases or parasites to which wild donkeys are prone, and some have been sold to breeders in Sardinia and beyond.
"I have no idea why I chose the asinello sardo – I'm just instinctively drawn to them," Giorgio told me. "They're generally very intelligent, and they interact well with humans. They can read our moods. I just knew I had to help them."
But the success of his programme is itself breeding problems. "Feeding the animals is now a full-time occupation, and the costs are mounting. I have to buy 2,000 bales of straw every year, but I get no financial assistance except the occasional donation from tourists. I must say the English visitors are very generous and usually leave something, but the locals don't even pay the ¿3 [£2.20] entry fee. Typical Genovese!"
I left the ramshackle Mazzuchetti ranch as its cheroot-smoking owner completed his morning rounds, hoping perhaps for a miracle to match the curious events associated with San Pietro's beautiful 19th-century church, Chiesa della Madonna dello Schiavo. This is dedicated to another significant statue, carved out of painted wood and known as the Black Madonna – the symbol of the islanders' release from slavery. It's said that the statue was found in 1800 by a wandering slave in Tunisia, who testified that it was hovering between two trees. After a papal blessing, numerous miracles have been attributed to it. As recently as two years ago, a child suffering from cerebral palsy reportedly left her callipers propped against the altar rail and walked unaided out of the church. And, yes, there's another festival in honour of the "virgin of the slaves" – on 15 November. I left the church, determined to leave my scepticism alongside those callipers.
San Pietro, where the people were rescued from slavery, where an endangered species is being saved from extinction, where old superstitions survive and thrive, and where normal life stops at regular intervals for joyous celebrations of food, drink, history, art and religion, is one of the unsung gems of the Mediterranean. And, to this dazed and confused traveller at least, possibly the strangest.
Cagliari is the airport for southern Sardinia. British Airways (0870 850 9850; www.ba.com) flies from Gatwick and easyJet (0905 821 0905; www.easyJet.com) flies from Luton. Car ferries (00 39 081 844 9297; www.saremar.it) sail to Carloforte from Porto Vesme and from Calasetta on the neighbouring island of Sant' Antioco, connected by bridge to Sardinia. The return fare is €14.80 (£10.60) per car, plus €6 (£4.30) per passenger.
The writer hired a car with Carrentals.co.uk (0845 225 0845; www.carrentals.co.uk). One week's car hire in Sardinia starts at £110.
The four-star Hotel Riviera, Corso Battellieri 26, Carloforte (00 39 0781 854101; www.hotelriviera-carloforte.com) has double rooms from €110 (£79).
Holiday Options (0844 477 0452; www.holidayoptions. co.uk) offers week-long packages, including return flights from Gatwick, transfers and B&B accommodation, from £739 per person.
EATING & DRINKING THERE
Ristorante Al Tonno di Corsa, Via Marconi 47, Carloforte (00 39 0781 855106; www.tonnodicorsa.it) is renowned for its tuna dishes.
The Pro Loco tourist office, Corso Tagliafico 2, Carloforte (00 39 0781 854009; www.isoladisanpietro.org) opens daily 10am-noon and 4-7pm in summer. Or try