Outside, the mist rolls in, rain lashes down on the bare green hills. As the lift machinery creaks slowly, ominously into the depths of the Big Pit in south Wales, the guide looks cheerfully round at our group. “Make sure your lights work, now! You’ll need them down there.” As we hastily fiddle with the three positions on our helmet lights, a young member of the party whispers anxiously, “How dark, do you think?” As dark as dark can be. There is, perhaps, no more wonderful moment in the wonderful hour-long tour underground than when the guide halts us in a long gallery, empty except for some old metal track, and gets us to turn off our helmet lamps. The silence presses down on our eyelids, heavy, implacable. “Can you see anything?” he asks “I think I am beginning to see something,” mutters the young person, hopefully. But she is wrong.

There is no gleam of light to enter here, 300 feet under layers of ore-bearing rock, no sound apart from our own breathing and the drip of water echoing from another gallery, no sense of space. All is dense, cold, impenetrable dark. The young person let out a cry; we turn our lamps on; we blink at each other with relief. It is just an old coal mine after all.

Now think of four-year-old children, left alone in rags in that darkness for eight, 10, 12 hours at a stretch, with rats biting at their toes, their only duty to flap the heavy ventilation doors which kept their older relatives alive as they laboured in the mines (say, 1840). Think of women chained to heavy carts, labouring uphill in that darkness (1850).

Think of sudden, terrifying panic as a cart tears loose from its steel hawser and careers down a gallery, smashing and crashing on its way, while the hawser whips wildly across anyone around (1880). Think of power failures, when the massive thudding generator stops and there is no lift out (1930).

Back to an earlier time, when individual miners climbed 300 feet down rickety wooden ladders and back again with baskets of coal on their backs, one slip meaning slow, agonising death at the bottom (1700). Think of methane, before the gas-sensitive Davy lamp, “Coal damp!” and a sudden fireball burning up the air and flesh (1800).

Think of rotten pit props giving way in the biggest mining disaster of the century (1920s) and a grinding, thundering collapsing slab of rock. And all of this in that total darkness.

The Big Pit, still rigorously inspected for safety every day, governed by government regulations about safety gear and machinery maintenance, is open for business as a museum. Pioneering in its day for its workers’ baths (opened 1937), with an ingenious mechanism for drying dirty clothes when miners were off-shift, it is equally pioneering as an experience of history in the raw.

Big Pit’s guides, all ex-miners, are authoritative on all aspects of mining, from the intricate machinery of the generator to the social history of the Depression. Whether you are crouching in one of the tiny openings on to the coal face where men used to work non-stop for hours on end, picking your way through puddles of dank water, or gazing up and down the metal tracks, they bring the long-gone working scene to life: the noise of pick and drill, the smell of sweat and the swirling dust, the ever-present sense of danger.

Perhaps the most touching are the pit-pony quarters. Here the beasts, their names still over their stalls, lived a life entirely away from light. Though the walls were painted white and there was special ventilation, it was not a healthy life and the ponies generally went blind. At the end of their working life, they would be winched back into the light. Some mines shut down for two weeks’ holiday, and the ponies used to go on holiday too. If you feel bad going back to work after two weeks’ lying on the beach, think how the ponies felt going back down underground after a spell chewing grass in a field.

A visit to the Big Pit is full of such startling glimpses into vanished worlds. All over the large site - you can visit the machine shop, the blacksmith’s forge, the battery store, the winch house, the baths, the canteen, and the sick room, and there is a place to eat packed lunches and listen to talks - the ghosts of past colliers seem to huddle in the damp air. Excellent audio-visual displays in the old canteen trace the history of mining in the area, the political thread which ran through miners’ lives, from angry notes pinned on doors of those who broke strike action in the 1820s to the 1930s news clipping about the opening of the baths and on to today’s transmutation of grimy industry to gleaming heritage.

But it is going down-under which sears the mind and draws out tributes to the bravery and endurance of so many miners. It also makes that very politicisation, so startling to the rest of us in miners’ industrial actions in the 1960s, 70s and 80s, completely real. When your escape from being crushed to death by tons of mountainside depends on solidarity, you get solidarity.

Emerging from the deep darkness with feelings of intense relief, the youngest member of the party spoke for everyone. “I’m glad I’m not a miner,” she said simply, handing back her helmet and battery pack before adjusting her pink hair slide and scampering off to the gift shop. And yet, of course, without those miners, without coal-fired factories and power stations, none of our amazing technology would have developed. More reason to commemorate those little children shivering as they struggled with air doors in the deep mines.

The Big PitPwll Mawr, Blaenafon, Torfaen, South Wales NP4 9XP. Tel: 01495 790311 for prices and dates. Book first. Safety: visitors must be minimum one metre tall