On the day I visit, the black slate of the Blaenau Ffestiniog mine contrasts sharply with a cloudless blue sky. It makes for an idyllic rural scene. And then, with a piercing whistle, a flash of red whizzes past overhead – a person suspended from a zipwire.

The weather is uncharacteristically good for North Wales in May. And it was the local rainfall – over three metres annually, more than twice the national average – that inspired the owners of Zip World Caverns to take the typically high-altitude activity underground.

Sean Taylor, an ex-Royal Marine Commando who grew up locally, first brought ziplines to a patch of woodland nearby in 2007. Now, he and his partners have constructed the world’s largest fully underground zipline course.

Harnessed up, I leave the light of day to trudge down a long tunnel led by my guide, Paul. With his short, stocky stature and an impressive ginger beard, it strikes me he could not be better-suited for the job. He doesn’t say much but when I suggest it must be strange to spend all day working underground, he turns to look at me. “The weather out there,” he croaks. “This is the best place to be.”

With untold miles of industrial-strength cable, and some clever lighting, Taylor and his team have repurposed the slate mine, which had been out of use for 140 years, into a novel tourist attraction.

Construction took 12 months and was completed without heavy machinery. The hole for each plug was drilled by a worker suspended from the rock-face.

Operations manager Simon Strevens (Colin McPherson)

“It has been known to rain every now and then again in Snowdonia,” jokes Taylor, whose 20-year military career would put Action Man to shame – including spells as a jungle warfare instructor, parachute display team member, and military ski instructor. “So we wanted an all year-round, all-temperature venue.”

Zip World Caverns is part of a wider wave of regeneration sweeping the area. Adventure tourism is the fastest-growing form of tourism in the world, and North Wales has rapidly secured its place on the map. From kitesurfing to dirt-biking and hill-walking, the sector brings in £481m to the economy every year, according to Wales Activity Tourism Organisation.

“It’s a good news story all round,” says Taylor. “We’re employing 240 people now and we pay them way above the living wage.”

The area historically has struggled with high unemployment, but visitor numbers to the mine (there’s also a museum and teashop here), are up 300 per cent. And Taylor estimates that the parent company – which also runs an underground trampoline complex here – has contributed £2m to the exchequer since 2007.

The course lasts roughly three hours – and is an experience at points both eerie and breath-taking. The main space is vast: you could pack 75 double-decker buses in floor-to-ceiling. Paul encourages me to stop and take in particular vistas. The space is accentuated by the dramatic lighting. The temperature never rises above 10C and my breath hangs in the air before me. Looking down at points, I struggle to see the cavern’s floor for darkness.