“No,” said the man in front of me, staring into the abyss below. “Just no. No, no, no.”
We’d chatted earlier this morning beneath the bald, foreboding peaks of north Wales’ Snowdonia Mountains. There, back on the surface, he’d seemed like a pretty happy-go-lucky guy. Here, 100m below, it was if the darkness of the mine had swallowed any light-heartedness whole.
He wasn’t alone. Even before we entered, on our mile-long walk uphill past enormous heaps of broken slate and several abandoned miner’s huts, we’d lost one of our eight-person group. It turned out she hadn’t fully realised what she was getting into.
In fact, none of us had – until now. We were tackling the Go Below Ultimate XTreme Adventure – a three-mile route through a massive slate mine that includes the world’s deepest underground zip line, first underground free fall, a 22m abseil and numerous traverses and via ferrata-style climbs, not to mention goes to the deepest underground spot in Britain that’s accessible to the public.
If that weren’t bad enough, we’d later discover that the mine, Cwmorthin Quarry, was nicknamed “The Slaughterhouse” by locals – thanks to the number of miners killed working there in the 19th Century.
Now, barely an hour into the daylong experience, we stood at the edge of a cavern. The next step would take us onto a wooden ledge the width of my foot. Below was a nine-storey drop.
The 4m-long ledge was followed by another, but we couldn’t see that far: the vertical rock wall – and the darkness beyond – made it feel like our guides were asking us to walk the plank into complete oblivion.
Meanwhile, I wasn’t sure what was more disconcerting: that we were 100m below ground, or that this depth was nothing in comparison to what we would do. Later in the day, we would reach a spot so far underground – 400m – that you could fit the tallest building in Europe, London’s Shard, between us and the surface of the hill above.
This, of course, is exactly what makes the route so thrilling.
Launched in 2015 after five years of planning, the 18-and-over Ultimate XTreme Adventure is the latest, and nuttiest, offering from Snowdonia-based company Go Below. Founded in 2010 by husband-and-wife team Jen and Miles Moulding, the company also has less extreme options, including a half-day adventure for those aged 10-and-up.
Whichever experience you book, the mine is not a show mine. Opened in 1810 and scattered with historic remnants like old railway carts, tallow candles and even a leather miner’s cap, it hasn’t been modernised. There are no electric lights, guardrails or even smooth flooring.
Instead, whether we were finding footholds in rock walls or tiptoeing around well-sized holes in the floor, we depended on the classic trifecta of climbing gear: harness, ropes and karabiners to clip ourselves into the fixed ropes that were bolted to the rock.
The only light came from our headlamps. At one point, we turned them off. The blackness that ensued was velvet-soft, so deep that I saw little sparks as my retinas tried to make sense of what was going on.
As intense as the experience was, we couldn’t say that we hadn’t been warned. “Many people who’ve done it have later said it was one of the toughest days out they’ve ever had,” the website says, urging that you not sign up unless you’re prepared for “a long day of scrambling, walking, climbing, zip lining and swinging on ropes” and adding “there are copious vertical drops which will test your nerve even if you’re not normally scared of heights.”
Still, I had thought, given that it was open to the public, how insane could it be?
I got my answer from Will Virano, one of our two guides: “This is the craziest thing you can do underground in Britain without special certification,” he said.
Of course, the danger we felt was mainly psychological. Everything has been tested, and is continually tested, to the nth degree. There’s an easy alternate route for every section, just in case someone panics – or is hurt – halfway through. And aside from some bumps, bruises and a re-aggravated knee injury, there haven’t been any major issues.
But it’s easy to forget all of that when you’re facing an abyss – which is where the other main safety factor comes to the fore: the guides. Will and our guide Danny Woodford-Phillips not only knew every twist and turn of the mine like it was their own living room, but – somehow – knew how to encourage us to take them on.
Like at that first challenge. As my fellow adventurer stood shaking his head, the guides quickly intervened and moved him to the back where Will spoke with him calmly, giving him time to decide whether to continue.
That made me next. Taking a big breath, I stepped onto the plank and edged along it. I slid the two karabiners from my harness along the rope above me. At the bolt, I clipped off one rope and secured it to the fixed line, then the other.
Halfway across, I did exactly what they told us not to do: I looked down. The beam from my headlamp bounced faintly off the rocks 27m below.
Danny’s words echoed in my mind: “You will find yourself beyond your comfort zone today. If you don’t, we aren’t doing our job.
Once on the other side, I saw that everyone was on their way; no one had decided to turn back. But there was no time to relax. Next up was a wall with only metal loops, followed by one with the occasional bit of stone sticking out. After that? A 22m abseil over the edge.
With each section we completed, another, somehow more challenging section seemed to come next.
The organisers had affectionately named our first zip line “Goliath”. The 130m-long cable runs across a gaping cavern, plummeting down 60m. The world’s first underground “zip ride”, it looked incredibly eerie: a wooden playground swing, dangling in the darkness from a 13mm cable. But once on it, it was hard to feel afraid. If I shrieked “wheeeee!”, I wasn’t the only one.
The next zip lines didn’t have so much as a seat. Instead, you clipped yourself into the cable and stepped off the edge. When Danny and Will explained that one of the lines didn’t have enough vertical drop to get you from one side to the other – meaning we’d each have to take a running start and hurl ourselves off the edge at speed – I started laughing.
Everyone else started laughing, too, when I clipped in and found myself, at 5’3”, lifted slightly off the ground – so that instead of taking a running start, I could only tap my tiptoes beneath me, their maniacal rhythm matched only by my increasingly nervous laughter.
So maybe there was no point at which we became completely comfortable. But that was the point.
At the day’s end, our final challenge was the free fall: a 22m plummet to a cavern floor. I readied myself at the edge. “OK,” I said, and waited.
“You have to take the step yourself,” Danny said patiently.
“Oh, right.” He’d told us that, but my nerves had already turned the memory of his instructions to mush. “Yes. OK. I’m going.” I stepped – and barely had time to register that I was hurtling to the floor before the brakes kicked in and I landed, safely, on my feet.
Was the Ultimate XTreme Adventure more terrifying than I’d imagined? Yes. But as it turns out, you can be scared stiff by something… and do it anyway.
More surprising still? You can have so much fun in the process that, as soon as you’re back on the surface – covered in dirt, your heart still pounding, feeling thankful to see the rainy Welsh skies – you’re already thinking about when you can return.
This story is a part of BBC Britain – a series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.
If you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.