IT IS a vast hi-tech subterranean cavern complex in the middle of a mountain, reached only by a kilometre-long tunnel blasted through solid rock, and it is owned by a company called Drax.

So far, so James Bond. But, surprisingly, 007 has never set foot inside this hideaway large enough to accommodate a seven-storey building the size of a Premier League football pitch with ease.

This is Cruachan, Ben Cruachan. The hollow mountain.

And it’s breathtaking, as I discovered during a visit last week while on a three-generation family break in the spectacular surrounds of Scotland’s Argyll & Bute. You won’t have seen anything quite like it before.

The underground machine hall of the Cruachan hydro-electric power station, on the shore of Loch Awe, lives up to its location – it is appropriately awesome.

The figures are crazy. The hall is 91m long, 37m high and 23m wide, houses four giant turbines, and sits almost 400m below the reservoir and dramatic dam high up on the mountainside.

Little wonder that when Star Wars film-makers wanted to created an impregnable fortress facility for the Rogue One movie, they set up camp at the base of the dam and started shooting.

How does it work? Well, you can discover the fine details here, but basically water released from the reservoir barrels down pipes to turn the turbines, spin the generators and produce power.

Unusually, however, Cruachan can also pump water back up from the loch to the reservoir, and, so the guides tell you, act as a storage facility for electricity. It’s recycling super-sized.

But, back down to earth, the good news is that the power station complex is open to the public, although only by prebooked guided tours because of the logistics involved.

So what’s the visitor experience like?

You enter via the Visitor Centre, which looks much like any other from the outside. Because you can’t see the dam up on top of the mountain, or the turbines deep below, the drama is hidden.

There are interactive information boards, video screens, scale models and memorabilia dating back to as far as 1921 when the scheme was first mooted. There’s a cafe and a gift shop, too.

Watch out for the Kidzone where younger visitors can learn, play and dress up in hard hats and hi-vis jackets. Owners Drax have done their best to offer an experience for all ages.

Be warned, however, that the machine hall – the jewel in the crown – is not accessible to wheelchair users or those with limited mobility. Given the space available and the different levels underground, it’s just not been achievable.

Arrive at the waiting area 10 minutes before your tour time and watch a video tracing the history of the plant – it’s very Pathe News style and with a plummy-voiced narrator – until the bus arrives.

The minibus carries 15 visitors, along with the driver and tour guide, and you’re required to wear seatbelts. First up, there’s a stop above the tailrace tunnel through which water reaches the loch, while you’re given an introductory briefing.

Then the excitement begins. The drive down the sloping 1km tunnel into the heart of the mountain is dramatic as you pass the overhead lights, peering out the front in search of the road’s end.

Eventually, you park up and file out into an atmospherically lit underground passageway alongside which scenes of the brave miners who blasted through the rock are played out on film.

There are several steps up to the Viewing Gallery, where there are more information boards and photos, including some of the day the late Queen Elizabeth II opened the facility in 1965.

But it’s the window at the far end of the gallery that draws you, perhaps aptly given the nature of the turbines below, like a magnet. From here, you get your first view of the cavernous machine hall.

I’m pretty sure that when I first visited Cruachan as a child with my parents that you could walk the floor of the hall but I guess health and safety has put an end to that. Still, the view is impressive.

The guide offers up more information, allows time for photographs – note that no video is allowed, and that bags must be left in lockers back at the visitor centre – and allows Q&A time.

Then it’s back to the bus, and back up the tunnel, which seems surprisingly steep on the return journey. By the time you step back out into daylight, around half an hour has passed.

It’s not a lengthy tour, but if you add in the exhibition, a snack and a coffee at the Hollow Mountain Cafe, and a meander round the shop, you’ll probably pass an hour or more here.

If the weather is fine, you’ll want to sit out on the cafe terrace overlooking Loch Awe. Just remember that if you’re visiting during midge season to apply generous amounts of jungle juice.

The Cruachan Visitor Centre is open Monday to Friday, from  9.15am to 4.45pm in summer (April to October), and from 9.15am to 3.45pm in winter (November to March).

Note that the tour does not include access to the dam itself , which can only be reached on foot, along a three-mile track from the nearest car park in Loch Awe village.

Admission costs are modest at £7.50 for an adult, £6.50 for students and seniors, and £2.50 for visitors aged five to 15. Under-5s get in for free. Blue Lights and Green Travel discounts are available – see the official website for more info and full details.