I was floating in the middle of the Atlantic Ocean, halfway between Brazil and Congo, a hair’s-breadth off the Equator, when I realised that Craig was yelling into his snorkel.

There were fish everywhere. Dozens of blackfish – the local name for black triggerfish – had been nosing around me, puppy-like, in search of snacks. Butterflyfish. Blue tang surgeonfish. A while earlier there had been a long, patterned barracuda, sliding through the silky water several metres distant and below me, focused – fortunately – on the rocks below it and not the wonder-struck human morsel floating above.

The surface was a cacophony, slicked with sun-warmed guano and broken feathers from the clouds of Ascension Island frigatebirds whooping and circling and chattering and fighting on the cliffs that powered straight up from the water beside me for thirty metres or more, festooned the whole way up with flaps and squawks. The Ascension frigate is one of the world’s rarest birds. This cliff-girt dot – Boatswain Bird Island, a cragged, forbidding rock off the coast of the cragged, forbidding rock that is Ascension Island itself – is the only place in the world where it nests, barring a few pairs on the Ascension mainland. It makes up for its absence elsewhere by virtually blotting out the sun here.

Ascension, a UK Overseas Territory, lies 4,200 miles from London and more than 700 miles from the nearest point of land, the oceanic pimple of St Helena. But it’s long been bound tightly into global routes of communication. When I visited earlier this year, it had long hosted a twice-weekly air link with the UK, via an RAF charter flight – open to civilians – that stopped midway to refuel on the journey to and from the Falklands. Last year 1,454 people visited Ascension as tourists, each one having to pay a £20 fee and be granted permission in writing by the head of the island’s government, known rather chillingly as The Administrator.

But since 15th April 2017, that refuelling stop has been diverted while repairs to Ascension’s runway take place, scheduled to continue until 2019 or 2020. On Ascension, everything has gone even quieter than usual. For now, the only way in is with the ageing Royal Mail Ship St Helena, which plies from Cape Town. Journey time is usually about six weeks, comprising ten days at sea each way, plus three weeks on Ascension between ship calls.

Is it worth it? Below the squawking frigatebirds I was discovering a bluer, quieter world of curious eyes and refracted sunlight, patrolled in the depths by lithe, tall-tailed Galapagos sharks. They are dangerous – people have been attacked in these waters, and Craig was nearby with a weapon should things have turned nasty – but these few were minding their own business, four or six or eight of them, longer than I am tall, sideswiping along the bottom. The current soothed me. My own rhythmic breathing through the snorkel was a lullaby. The scenery blurred into a psychedelic forest of colourful bodies and vertical tails, highlighted in shifting bars of dappled yellow.

Then Craig was yelling at me underwater and jabbing his finger. Yelling. Jabbing. I turned, too slowly, to see a big grey body, up at the surface, near us. “It’s huge,” I thought, stupidly. For a beat of time, I heard silence. And then behind the body wagged another kind of tail. A flat, horizontal tail. It took my sea-addled brain a second to compute. It wasn’t a fish tail. It was the tail of a cetacean. Craig was yelling “Dolphins!”

I met three bottlenoses that day. The pod was much bigger, but most of them were too far off, and heading elsewhere. My three were scouts, or outlaws. They gave me side-eye, and one, two beats of those muscular tails, and then there was only infinity. I bobbed up through the scummy surface, climbed onto the deck of the Argonaut and let the sun dry my heavy head.

As we chugged back to Ascension’s miniature capital, Georgetown, Craig Hall – born into a fishing family near Durban, now proud owner of the Argonaut and his own company, Ascension Island Adventures – cast his lines and told some stories about how the island is, in his words, the best fishing destination in the world.

“People get excited about marlin, but nowhere else can you catch 100kg tuna offshore,” he said. “I’ve seen it myself, a 100kg tuna in five metres of water.”

Sharks, rearing up behind the boat, got two of the smaller tuna he caught on our return journey, but eventually he landed one, maybe 15kg.

“I had one once with 29 blackfish whole in his belly, and another sticking out of his mouth, and he still took the lure,” laughed Craig, as he pulled his supper onboard.

I don’t think he was trying to impress me. He just wanted to share how great his island is.

Ascension emerged above the waves only a million years ago, the summit of an undersea volcano. Contorted flows of black lava ring the coast, looking like they cooled yesterday. Red cinder cones stud the interior.

Until humans found it in 1501, there were only seabirds and turtles and a few tiny ferns. But in 1815, Napoleon lost Waterloo and was exiled to St Helena. Fearing French attempts to spring the Little Corporal, the Royal Navy garrisoned Ascension.

Fresh water was scarce. Darwin visited, then botanist Joseph Hooker. The two discussed how to make the island more habitable.

Thanks to warm southeasterly trade winds, clouds often envelop the highest point, Green Mountain (859m). Hooker proposed planting trees on the summit to trap the moisture, letting it drip down as water for the troops. He would introduce pasture for livestock, and soil for vegetables.

It was a startling success. Over decades from the 1850s, Hooker created a global smorgasbord of plant life. From the lava plains, baking in 30C sun, I drove a switchback road to stand beneath mature dripping fig trees on a mist-wreathed shoulder of Green Mountain, breezy at 22C. I walked beyond them to the summit, skirting groves of guava and ginger, ascending through creaking bamboo and passing a thousand other species, none of them native. Green Mountain is the world’s only artificial cloud forest, dense and Narnia-like – an environmental disaster (alien species have all but swamped the endemic ferns), but a botanical wonderland.

Conservationists long focused on trying to beat back the invasives, particularly the mesquite known locally as “Mexican thorn”. This was introduced in the 1960s by BBC engineers brought over to build a shortwave relay station. It’s since run riot, tens of thousands of spiked warriors smothering the island, almost completely resistant to human intervention. Scientists who’ve tried to dig them up have discovered tap roots more than 20m deep.

While more radical plans to take back control develop, conservationists are switching tack to encourage human access. In the last few months Andrew Airnes, the local reserves warden, has cleared undergrowth from Elliot’s Path, a 3.6km trail contouring around Green Mountain below the peak.

“It had been left to its own devices for years,” he told me. “I want to start embedding the value of conservation for people.”

Elliot’s was created in 1840 when the mountain was still bare, to give 360-degree views over the whole island. Those stupendous views survive, through windows cut in thickets of ginger, or out across now-wooded cirques, mist racing below. Halfway round I rested on logs of Norfolk Island pine that, as telegraph poles, carried the message of Neil Armstrong’s moon landing from Ascension’s now-defunct NASA station, visible on a distant hilltop. Pairs of fairy terns – spectral white, with zombie-black eyes – cartwheeled up out of the ravine to hang, momentarily, in front of my face.

Down in Georgetown, a few asphalt roads thread between aprons of black clinker. The thing about Georgetown is the noise. There is none. Plaster flakes. Behind the till at A&P’s Gift World, Amelia Knipe, bare-armed and motherly, laughed with her customers while I fingered colouring books and tins of spaghetti hoops.

“This place isn’t supposed to be known about,” said Jolene Corney, rearranging cartons of sweets in the Movie Shack DVD rental store.

Ascension has forever been dominated by the military. It’s British – the BBC still broadcasts to Africa from here – but the RAF shares the airfield with the US air force, and the island’s strings are pulled far away, in London and Washington. It’s not always obvious who calls the shots. Most of the 850 residents are on temporary contracts, with women outnumbered two to one. NASA still has a presence, and the island is one of four global nodes in the GPS navigational system. Satellite dishes and elaborate radar arrays decorate almost every hilltop, though it’s unclear who is listening to what.

One sunset, I joined a sandalled crowd beside the European Space Agency monitoring station at Northeast Bay to watch the spawning of Ascension’s land crabs. They come down from Green Mountain in spring to mate and release larvae into the shallows. Cloaked in twilight, thousands of the yellow monsters covered the rocks, darting in and out of the waves and the beams of phone torches.

By midnight the crescent beach was in darkness, and crabs still skittered from the sea to the mountain, and great heavy shadows of turtles hauled themselves out of the surf to lay eggs in the sand, and the ghost of Leendert Hasenbosch – a Dutch sailor abandoned here in 1725 as punishment for homosexuality, who left a desperate diary of bird-eating and blood-drinking – stood at my shoulder, and Orion lay prostrate over the booming ocean, and nobody spoke, and I thought of all the places we know nothing about, and I felt lucky.