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AS I passed through a set of iron gates built into a forbidding slate hillside, and was told that I was entering the underground halls of Mount Elidir, it was hard not to feel a little bit like a character from The Lord of the Rings.
The strangeness of the moment was not lost on a group of Kansas City teenagers. "Wow!" cried one girl as our minibus drove into the tunnel. "This is so spooky!"
"Right!" said her friend. "Do you think there'll be dragons?"
In a way, there were - if you count the monstrous, green-painted turbines that were roaring deep inside the heart of the mountain, kept in a 500ft-long chamber that was half cathedral nave, half outsize sports hall.
"You are now", shouted our guide, "at the very centre of Electric Mountain, fifteen hundred feet below the summit, and three-quarters of a mile from the gates we came in through." Plus another half-mile from Llanberis, the North Wales village which is the traditional starting point for most Snowdonian adventures - except that this expedition involved going down rather than up.
The name of the beast we had come to visit was Dinorwig, an underground hydroelectric power station built 15 years ago. It is, as physics teachers will testify, no easy task making volts and megawatts interesting. The main drawback is that electricity is invisible - and therefore somewhat less than absorbing to look at. Even our pre-tour, audio-visual presentation had to be augmented with lots of strobe-lighting effects to hold everyone's attention. Where Dinorwig was different was that even the most unscientific among us were able to grasp the simple but spectacular way in which the place worked.
What happens is that when a sudden, national top-up of electricity is required - such as at half-time during a big, televised football match - the plug is let out of a dam at the top of Mount Elidir. Millions of gallons of water then pour down the pipes inside the mountain, sending turbine wheels spinning and producing electricity within 10 seconds (it can take a coal-powered station 12 hours to stoke up). Once it has done its work, the water is piped to another dam - this time at the bottom of the mountain.
"So isn't the top dam, like, empty?" asked a Kansas boy. "Or do you guys just wait for the rain to fill it up again?" This was by no means an unreasonable assumption, given the torrential downpour outside. The truth turned out to be even stranger: once the water has flowed down to the lower dam, it is pumped back up to the top of the hill again.
It all seemed a bit Grand Old Duke of York-ish, until our guide - a smartly-uniformed local woman called Jean - explained: "We carry out the pumping in the middle of the night - at off-peak rates. Usually between two and six in the morning, when everyone's asleep."
"But why did you have to build the power station inside the mountain?" asked another member of the party, after hearing how the construction took 10 years, cost £450 million and involved the scooping-out of 12 million tons of slate.
"For environmental reasons, obviously," replied Jean. "After all, we are in the middle of the Snowdonia National Park."
It is, however, a surprisingly ugly part of the park. Electric Mountain (Mynydd Gwefru in Welsh) already bears the unsightly scars of 200 years of slate mining. The surrounding hillsides have been so deeply gouged that they look like black-and-white photographs of bombed cities in the Second World War.
Thirty miles away, at Blaenau Ffestiniog, the industrial debris was even more in evidence. What looked from a distance like a vast amphitheatre full of football fans, turned out to be another ravaged escarpment, peopled not with faces but with hundreds of thousands of shiny slate slabs. In its shadow stood Ffestiniog, another power station with water in its veins. Again, there was a dam at the top of the hill, a dam at the bottom - and, for added effect, a narrow-gauge railway in between.
Here, our underground trip was undertaken on foot, some 100ft down a steep series of stairs and into the station's innards.
"See that wall?" said our guide, as we reached the lowest point and removed our earplugs (the machinery was very noisy) to hear her. "That's all that stands between us and the dam. If water starts coming through, we retreat through that little door and shut it behind us. It's watertight, so that if one half of the station floods, the other will stay dry."
With a rather more hands-on tour than Electric Mountain, the journey to the depths of Ffestiniog was like exploring the below-water-level workings of an ocean liner - lots of solid, capstan-like objects, all very shipshape and painted bright blue, and full of reassuring notices such as "No. 3 Unit Governor". There were plenty of busy machines to peer into as well, with see-through, twin oil chambers cooking up what looked like chicken broth in one compartment and lentil soup in the other.
Physically, both of the cavernous and super-efficient power stations bore a marked resemblance to a baddies' HQ in a James Bond film. However, despite their awe-inspiring dimensions, each place has fewer than 30 people working underground - even fewer on some occasions. "Come weekends and Bank Holidays, this station is run by just two people," revealed the Ffestiniog guide nonchalantly. "There's automation for you."
Should either station ever need extra staff, there is a chance that they could be supplied by King Arthur. According to legend, he and his knights are said to be sleeping in a mountain nearby, on constant alert should their country ever be threatened.
For the moment, these twin kingdoms of Dinorwig and Ffestiniog are able to make sparks fly without his help. Although the constant draining and replenishing process means their tidemarks go up and down quickly, neither place is likely to see its stream of visitors run dry. After all, on a rainy day in North Wales, where better to go than an underground power station run on water?
Electric Mountain, Llanberis, Gwynedd (01286 870636). Open daily until Dec 23 and from Jan 4, 10.30am-4.30pm. Tours at 11.30am, 1pm, 2.30pm, 3.30pm; £5 for adults, £3.75 for pensioners, £2.50 for children; booking advised. Ffestiniog Power Station, Blaenau Festiniog, Gwynedd (01766 830465 or 830310). Open March to Oct, 10am to 4.30pm; tours, adults £2.75, pensioners £2, family ticket £7.25, £1.50 children.