Our boat's engine splutters to a stop and we peer over the side in silence. This is Spot X, a dive site with no name or listing in scuba guides to the Azores. Well, so I'm told. Actually, we're some distance off Sao Miguel's black basalt cliffs, our little vessel rocking to the long ocean swell and there's no indication of a dive site at all. There's no island nor rock pinnacle. Not even a hint of seabed despite visibility of more than 30 metres. Spot X seems to be nothing more than coordinates on the GPS.
Most unnerving of all is the water. Instead of the usual turquoise, it's the Quink-shade of deep ocean. "I've never reached the bottom on this dive," Sofiane Belkessa says cheerfully.
Poor Sofiane. This was the third day that the owner of Nerus Dive Centre had been stuck with me. I'm the first visitor on Regaldive's advanced open-water course in the Azores and by chance I've come at the same time as his French dive buddies. So, while they marvelled at the sea life, he had watched me swim in triangles for the Professional Association of Diving Instructors (Padi) underwater navigation module. While they sank post-dive beers at Ponta Delgada marina, he quizzed me on Padi's homework or gamely assessed wonky images for my photography module, the diving equivalent of a scout's house-keeping badge. It was probably dives such as Spot X that made his job worthwhile. And now I was getting the jitters.
Still, we had taken half the morning to get here. No turning back now. Cocooned in 6mm wetsuits, we drop overboard. This is what astronauts must feel like, I think, as we drop weightless into the void, exhaling silver saucers to the surface. Ten metres, 15, 20. Still we fall, the only sound is the alternate hiss and rumble of my breathing. Then it appears: a long curve which arcs into the blue gloom. An edge to something huge. For a crazy moment, I think of spaceships.
We reach the rim of the volcano at 33 metres and drop into the crater where moray eels gape. The sides plummet to depths that I don't allow myself to think about, so I look up, just as three manta rays soar across the splintered sunlight like an outré sci-fi. Welcome to the Azores, where even the fish look extraterrestrial. This Portuguese archipelago is often hailed as the best diving destination in Europe. It hides in plain sight, 1,500km from Lisbon and 3,900km from New York; a mid-ocean roadhouse for Atlantic and Caribbean species alike: sardines, octopus and moray eels, but also loggerhead turtles, parrotfish, barracuda and manta rays. And because the nine islands spike almost vertically from the seabed, the world's largest pelagic species are found inshore: blue sharks, oceanic whitetips, spotted and bottlenose dolphins, and a third of the world's whale species.
Once hunted, the latter are now tracked on popular whale-watching trips from May to October. Factor-in volcanic landscapes, excellent visibility (no river run-off clouds the seas) and warm water of 16C to 22C (OK, warmish) and you have diver's Nirvana.
If few people realise all this, then the reputation of the Azores for pensioners' holidays is to blame. You can see how it appeals to cautious holidaymakers. It has a Goldilocks climate – not too hot, not too cold – and a bucolic exoticism. On Sao Miguel, the largest and most accessible of the islands, the verges of hawthorn-hedged lanes bloom with agapanthus and ginger lilies. Around the pretty Portuguese villages cows graze on calderas. It is a tangential reality, simultaneously familiar and fantastic.
Azores tourism has traditionally been sedate: bathing in geothermal pools; visits to pineapple plantations; even the whale-watching is ultimately passive. Yet the words above Nerus Dive Centre summed up Sao Miguel better: "Discovery. Adventure. Fun." With reputedly Europe's cleanest air, the islands now promote hiking and mountain-biking. It seems emblematic that young Scandinavians now come in droves and that Quiksilver and Red Bull choose Sao Miguel for annual surf and high-dive contests.
"Nature and the sea have always been a central part of life here, a part of our souls," my guide says. "And we want tourism," she continues, then adds quickly, "but not like in the Canaries," as if I will develop the rash idea that the Azores, with only one direct flight a week from the UK, might go mass-market.
Diving fits comfortably into the rebrand. Experience the raw ocean dives of the Azores and everywhere afterwards seems like a tepid tropical aquarium, I'd been told. All very exciting, but also troubling for a beginner. "A lot of people say, 'But you're in the middle of the ocean, it's deep, there are a lot of currents, it must be dangerous,'" says Pedro Jorge Piteira, of Azores Sub. "But there's a dive for all abilities here: technical, very deep, shallow, wreck dives, cave dives, easy dives with thousands of fish." To prove it, he takes me for a refresher in the sunlit shallows off Vila Franca, the island's former capital. It turns into a primer on history – we see old galleon canons and the "kitchen" where whalers once butchered their kill.
Yet diving here is in its infancy. Sure, Pedro wants to develop it, but not "like in the Red Sea". The lack of operators lets Sofiane pick from the sites around Sao Miguel's pocket-sized capital Ponta Delgada, where Nerus Dive Centre is based. A wreck-dive such as the Dori, 15 minutes from the marina, would be hugely popular in the Red Sea. Instead, we're alone. Well, alone except for carousels of fish by the ship's bridge, torpedo-shaped barracuda, hundreds of alien-eyed octopuses, fireworms like Rio carnival dancers and a stingray the size of my hotel's king-size bed which ripples off the deck.
What really wows is the sub-aquatic scenery. In Iceland, the Mid-Atlantic ridge produces landscapes to make your teeth rattle. In the Azores, that tear between the world's largest tectonic plates results in dives such as Baixa das Castanhetas in the Caloura Marine Reserve. Here, lava flows have constructed a city of monstrous arches. Pressed against the sand, we slip under one to enter a domed chamber bathed in slippery light.
I'm so surprised I barely blink at the miniature rubber dairy cow which crawls up a wall. Little thanks to my sketch of it for Padi's fish identification module, I find later it was some sea slug. Unbeknown to me, I was also being assessed by Sofiane to see if I was up to Spot X.
It turns out my taxi driver back to Ponta Delgada airport, a diver himself, also knows Spot X. In 1811, it boiled into existence as an island, he says. The captain of a passing warship, HMS Sabrina, claimed sovereignty for the British Crown, causing a diplomatic row only extinguished when Sabrina Island obligingly returned beneath the waves. He'll take me next time if I have a Padi qualification.
Of course, I say. Advanced.
SATA (00 351 296 20 97 20; sata.pt) flies each Saturday from Gatwick to Ponta Delgada on Sao Miguel island, until 19 October.
James Stewart went with Regaldive (01353 659 999; regaldive.co.uk) and Sunvil Discovery (020-8758 4722; sunvil.co.uk). A week's B&B at Hotel Talisman in Ponta Delgada costs from £649pp, including flights and transfers with Regaldive, which also offers Padi courses from £285 or a four-dive package from £109.
Sunvil has a week's twin-centre stay at Hotel Talisman and Hotel Caloura in Caloura, for £691pp, with flights, transfers and breakfast.