The beautiful corner of Thailand that tourists haven't discovered yet

A cluster of mysterious standing stones loomed ahead. Six thousand miles by plane from the UK, followed by a taxi ride, a speedboat and a longtail, and I’d arrived… at Stonehenge.

There are a few striking differences between England’s famous prehistoric monument and this henge out in the Andaman Sea, near the Thai island Koh Lipe, though. For a start, these granite pinnacles are 59ft beneath the ocean surface, coated in white and purple soft corals, and surrounded by thousands of golden damselfish and bigeye snappers. There are no coach loads of underwhelmed tourists clicking away with cameraphones by the side of the A303, either.

In fact, my dive instructor and I aside, there was no one else at all.

It’s not just crowd-free under the water; Thailand’s off-the-radar, south-west corner of Satun, on the border with Malaysia, receives few international travellers, a fraction of the hordes that hit Bangkok, Phuket and Koh Samui each year. If you know where to look, you can even get that rarest of things: a paradise Thai beach all to yourself.

Satun’s anonymity might be changing. Sprawling across four districts of Satun province, it was recently awarded "Global Geopark" status by Unesco, having been marked out as an area with geological heritage that is being protected and used in a sustainable way. There’s evidence here of an ancient sea landscape dating back more than 500 million years, with fossils of early organisms, as well as colourful rock formations, mountains, beaches and giant caves.

Locals have embraced their geopark status. We visited Panya Batik on the mainland, a community-run project producing batik (wax-drawn and colourfully dyed) textiles featuring trilobites, ammonites and other "locals" from back in the day, while Muslim women from the region served us khao tom (blue sticky rice) and other treats usually eaten at post-Ramadan parties. Even the basic hotel where we stayed for a few days on the coast had put up signs with "catchy", fossil-related room names: Tentaculite, Stromatolite, Ordovician…

We’d started, though, at Koh Lipe, the tiny island that is Satun’s tourist epicentre, packed with hotels and resorts (not allowed on the other 50 islands within Tarutao National Marine Park), beach bars, dive shops and a little walking street (imaginatively called "Walking Street"). Dropped off by speedboat, we set off one morning from Idyllic Concept, our resort on Sunrise Beach, to hop around the Andaman Islands in a smoke-spitting longtail. Anchored off an empty beach at Koh Adang, we snorkelled over an array of gold, purple and green coral gardens, accompanied by clownfish (of Finding Nemo fame) and neon damsels, while giant clams below waited for something to clamp down on.

At Koh Rawi, we shared the water with puffer fish, bright blue starfish, sea cucumbers and parrotfish that nibbled at spiky staghorn coral and funnel coral (like giant rose petals).

Later, we strolled along a beach of smooth black stones on Koh Hin Ngam. I got the subtle feeling here they don’t want anyone stealing rocks as souvenirs. A sign warned of "The Curse of the God of Tarutao Island", promising anyone who removes stones will face "Fatal accidents; Family life shattered; Loss of property and fortune; Loss of life." Like I said: nice and subtle.

I headed out from Sunrise Beach again next day with Max Ravi Gitsanalow from Satun Dive. Our first scuba dive offshore from Koh Adang rushed by in a blur, carried along on a fast-moving current; I spotted behemoth titan triggerfish and tiny nudibranchs, while keeping a beady eye ahead in order to avoid smashing into rock or coral.

Between dives, we dined on shrimp curry on one of Adang’s empty beaches, down from the bamboo home of Orang Laut sea gipsies who live and fish around these islands. "At somewhere like Koh Phi Phi, you might get 10 big boats at one dive site, maybe 100 divers swimming under the water," Max laughed, comparing diving here with some of Thailand’s better known hotspots. "Not here. It’s much more relaxed."

With stormy waves rocking the boat, we rolled backwards into the ocean and descended. Soon, we were surrounded by countless thousands of snappers and damsels, the sheer numbers a mesmerising sight. We swam through them to Stonehenge, where there was, indeed, a resemblance, though probably not enough to inspire its own Spinal Tap song. Some of the granite menhirs were standing in clusters, others lay flat or at angles.

We explored sprawling table corals, golden fan corals and big vase corals, which looked like the ominously hatched eggs from the Alien films. There was a notable absence of druids, pagans, hippies or ravers, but we were never short of company: bug-eyed bronze soldierfish, half-moon triggerfish, sharpnose pufferfish… Max shone his torch on a moray eel between two standing stones, snapping its jaws at passing fish. Making our way through a bank of white-belly damsels, I couldn’t recall another site I’d dived in Thailand with such remarkable corals and volume of fish, without any other divers to be seen.

Next day, we took a speedboat across to Tarutao, the heart of Tarutao National Marine Park. Quieter and emptier than Koh Lipe (though with many beaches in need of a serious clean-up of bottles, plastic bags and other rubbish), there are just two taxi "trucks" on the island, most people getting around by bike, moped or on foot. A wild boar wandered through the trees, while a cluster of oriental pied hornbills picked berries in the branches above.

Oddly, this little getaway, three miles (5km) from Langkawi, used to be a penal colony, established in 1939 by the Thai government to detain criminals and political enemies. At its peak, in 1941, 3,000 prisoners were held here. Life was not a beach. "They had to work hard every day," our guide Mukta Langkart explained, as we walked on the east side of the island. "It’s a paradise for people now. But for the prisoners it would have been more like a ‘Hell in paradise’."

Life became so desperate on Tarutao during the Second World War, when the island was all but forgotten and food and medicine became scarce, that the prisoners and wardens combined forces to become the "Pirates of Tarutao", raiding ships passing from Penang, Langkawi, Phuket and Myanmar (formerly known as Burma), stealing cargoes and killing crews. Eventually, the British government and Thai forces restored order, with the prison closing in 1948.

We stayed in no-frills (but certainly not prison-like) beachfront bungalows in Mo Lae Bay, with electricity for just a few hours each evening, no air conditioning (only a fan), no hot water and certainly no Wi-Fi. There were more vervet monkeys feeding and grooming here than other tourists, with waves crashing on Ao Molae beach, ours alone to enjoy.

A longtail boat pulled up next morning and the captain took us to Tarutao’s untouched snorkelling sites. Later, we picked up kayaks at Tarutao’s pier, off Pante Malacca beach, and paddled down the wide river through mangrove forests, their silvery trunks and spidery roots reflecting in glassy water. Flying fish broke the surface, while crested serpent eagles soared above towering black cliffs. Beyond that, only silence.

Back on the mainland, staying near Pakbara Pier, we took a walk through the "Time Traveller’s Zone" in Petra National Park. At the centre of the coastal walkway, there’s a meeting point in the cliff of red sandstone from the Cambrian Period (541-485 million years ago) and the limestone of the Ordovician Period (485-444 million years ago), clashing together, a jagged abstract artwork in gold, pink, orange, white and black. "An earthquake brought the two periods of rock together," Mukta explained. To walk past the fault plane, goes the spiel, is to cross 100 million years of geological time in just a few steps. It’s a popular spot for couples now, celebrating "love that straddles time".

We explored nearby Khao Yai by longtail, Mukta leading us up to a summit where nautiloid fossils are set in the stone, scientific evidence of life from 470 million years ago. Or, as a Creationist might put it, a trick planted by the Great Deceiver (Satan). "The fossils and geology are the number one reason Unesco protected this geopark," Mukta said.

We climbed into kayaks and paddled past caves where the Pirates of Tarutao stayed during raiding missions, then through a tunnel into Prasat Hin Panyod (meaning "1,000 Stone Castles’), a roofless sea cave crowned with jagged formations. A monitor lizard stalked the rocks at the exit, picking off insects with its blue, forked tongue.

On our final day, we drove inland from Pakbara to Phu Pha Phet Cave, or Diamond Mountain Cave (named for the glittering particles inside), the largest cave in Thailand and the fourth largest in the world. A narrow tunnel led into the first of 20 limestone caves, stretched across 215,300 sq ft, where prehistoric humans are said to have once lived. Inside, spider webs hung on metal lamps that creaked and swung with the movement of the wooden walkways. But it was a wonder to walk through, with staggering subterranean "room" after "room" filled with giant stalagmites, stalactites and columns where the two have fused together over thousands of years.

We were alone inside – no "Same Same" T-shirts, no selfie sticks. If such a natural wonder was anywhere near Bangkok, Phuket, Koh Samui or any other hotspot where the bulk of Thailand’s 35 million annual visitors cluster, it would have been rammed. Except for water and our footsteps, it was silent.

We reached the lowest cave, sunlight seeping in from a hole above on to a bright green rock, the so-called Jade Stone, which looks like a Star Trek prop for an alien planet. From there, we retraced our steps, past weird mounds, like jellyfish or ghosts, and other formations that looked like giant candle drippings, towering columns, "curtains" of calcite crystals, walls that appeared to have been smattered with icing sugar, human faces, a serpent’s head…

Just one reminder after another, here in Satun, of the wonders nature can produce if you give her a little time.

How to do it

Ethos Travel (020 7284 1888) offers a nine-day Satun Explorer trip in Satun Province, including three days on Satun mainland and four days on Koh Lipe, from £1,485 per person. The price includes flights from London Heathrow to Hat Yai, via Bangkok, transfers, breakfasts and some meals, tours, entrance fees and an English-speaking guide. Ethos can also arrange stays on Tarutao.

British Airways (0344 493 0125) has daily direct flights from London Heathrow to Kuala Lumpur, with returns starting from £442, including taxes and fees. Connecting flights from Kuala Lumpur to Hat Yai with AirAsia start at around £62.

For more on Thailand, see tourismthailand.org