The car park beneath the track was full and the rain was lashing down as I splashed through deep puddles to meet the Friends of Cwmorthin.

These volunteers from across Britain share a passion for mines - exploring, restoring and preserving their infrastructure, making them relatively safe places to visit.

Safety is an odd word for a mine whose nickname is Slaughterhouse. Many miners died here in the late 1800s due to bad management and in particular 'pillar robbing' - cutting into the pillars of rock that take the weight of the mountain above.

Ultimately this caused the great fall of 1882 when six million tonnes of rock collapsed into the chambers of this and neighbouring mines on the edge of Blaenau Ffestiniog.

I hung up my waterproofs to dry, donned a caving helmet with head torch and followed the Friends to the main entrance, the lake level adit.

An adit is a horizontal entrance to a mine, with pit props and steel girders holding back the avalanche of slate above. The collapse of this entrance a few years ago had been the catalyst for the formation of the group and this had been their first project - clearing debris and installing strong supports.

At 5' 8" it's rare for me to bang my head on the ceiling, but it happened here and I was grateful for the helmet. Inside the mine the normal peace and tranquillity was broken by a generator throbbing away, but the compensation was being able to see the wonders inside - sights the miners of Victorian times could only imagine with their limited candlepower.

We walked down an incline alongside rusty rails on which trucks would have been hauled up with their heavy loads of slate. The cardinal safety rule in any mine is to keep miners and wagons separate and an upturned wagon wedged tight between the floor and the low ceiling was a reminder of the need.

At the bottom of the incline we entered a vast chamber 150 feet high with a smooth roof of slate at an angle of about 45º. Sounds that had been muffled in the confines of the tunnels were now amplified, with drips of water and scuffed stones echoing round.

Rising up from the floor of the chamber was a stairway with steps cut out of rock and further up steps of wood or slate perched on bars of iron drilled into rock.

Today's job was to was to reinstate this route, with new steel pins six inches into the rock supporting thick slabs of slate and with a handrail of rope.

We climbed the miners' steps back to the lake level, followed a route downwards and then along a level tunnel into the top of another chamber with a derelict bridge spanning the void to the outgoing tunnel.

Just two massive beams remained of the old bridge, looking like something Indiana Jones might ride a slate wagon across at high speed, but we enjoyed the view and turned back.

Yet further into the mine, away from the sounds of the generator, I was chatting with the Friends of Cwmorthin about the work being done, the complexity of getting insurance and access permissions, when out of the darkness came whistling and footsteps.

It was the Oread Mountaineering Club from Derby, up for a weekend walking and enjoying dry refuge exploring Cwmorthin.

The mine is open to anyone, not just the Friends of Cwmorthin, and if you ask for the combination code to the gated entrance you will be given it. But you are expected to follow safety procedures, wear the right equipment, have a qualified mine leader and an underground map.

The tunnels, chambers and inclines go on for miles with just one entrance, the lake level adit. We walked out through this and up to smoke flue adit.

This is an arched tunnel lined with smooth slices of cut slate blackened with soot and a chimney at its outward end, where the smoke was funnelled out from the steam engine and boiler that once powered the old mining machinery.

About 100 feet into the tunnel there had been a mudslide which is about to be dug out to provide a second access point.

My guides invited me back to their temporary base where damp logs in the grate had given up their glow and the incessant rain was flowing freely down the back wall.

A kettle was boiled on a Primus stove and tea served in thermal mugs. Sleeping bags, provisions and wet clothes were dotted around the room.

The outside door was closed, but a breeze still found its way down from the patchy roof and up through holes in the floor. The tea was great, but I was glad I'd not been invited to stay for supper.

If you'd like to visit this amazing underground world see their website (link right). An open day is provisionally set for 15 November 2008 (over 18s only). Huw Jenkins

Listen to more about the Friends of Cwmorthin from the Roy Noble Show on BBC Radio Wales.