Beyond Ben Alder, deep inside the lonely grey mountains which lie to the west of Loch Ericht, there is a singular place. Persistent rumours tell of a cavern higher than the greatest Gothic cathedral, with a nave double the span and several times as long. Just like Clunie's Cage on the slopes of Alder, it's well hidden and rarely talked of. Existence is conferred by more than simply an appearance in magazines or books, but in the case of the Monadhliath 2 powerhouse, it is an absence. It doesn’t “exist”. Not officially, at least. Instead, it seems to have become something mythical, a cavern into which Lewis Carroll might have peered.
It was built during the era of Cold War paranoia, the 1980's, as a power station which would be called upon after Zero Hour, after much of the country’s generating capacity had been destroyed by an exchange of nuclear missiles. Whereas many power stations are built on the surface, and offer easy targets, hydro generation can be hidden. In this case, the intakes lie deep underwater in a remote hill loch: the giant penstocks and the powerhouse are carved out of the heart of the Grey Mountains.
Monadhliath exists rather like the Norse myth about the great wolf Fenrir, who the gods believed would bring about the world's destruction. They caught the wolf and locked him in a cage, but he broke free of every iron chain they shackled him with. Eventually, they trapped him, and he was chained to a rock a mile underground where he awaits the end of the world. When the End arrives, he will break free from this prison, too, and devour the sun.
The Monadhliath hydro scheme was first proposed in the 1960’s, when the Mackenzie Committee reported on future prospects for hydropower in Scotland. The odds of Monadhliath being constructed grew longer as time went on. After the Cruachan scheme’s completion in 1965, the North of Scotland Hydro Board’s great Development Plan appeared to grind to a halt. Opposition to future hydro-power came from landowners who resented the people of Scotland benefitting from our own mountains. Yet work at Monadhliath carried on, and a myth slowly grew around it.
The powerhouse is a mighty place, on a scale unparalleled by anything else in Scotland. Yet it was the first man-made thing on these mountains: before the dam, the top reservoir was just a lochan of shallow, peaty water, overshadowed by mountains. Few humans had been here. The area around it is a jumbled mass of rocks, studded with moss and lichen, crossed by foaming burns. 
The powerhouse was hewn from solid grey gneiss, lined with concrete and enamelled steel panels, a mystery hidden at the head of a remote stalker's track. Larger than its predecessor at Ben Cruachan, larger than Dinorwic in Wales, larger than any of the Snowy Mountains power stations in Australia, Monadhliath is a modern wonder. Ben Cruachan was a pumped storage scheme which utilises cheap night-time power to drive water back into the head reservoir after it has flowed through the turbines at peak time, as opposed to a conventional one which uses the water only once. Monadhliath develops that principle further.
Monadhliath, according to the Mackenzie Report of 1961, was to be a scheme of two stages, the first of which would total 150MW output but in the event, it's supposed that the output is much higher, since this station would be used over a short timespan, perhaps just a few hours, as a last resort. At Monadhliath 2, six turbines of perhaps 500MW each run at far higher power, but a far lower load factor, than originally anticipated by the 1960's era Hydro Board.
The size of Loch Ericht, almost 20 miles long, and the fact that other hydro schemes control both its inflow and outflow means that large fluctuations in level due to the secret power station can be absorbed, un-noticed. It is likely that the controls are all thermionic valve-powered: rather than being a step backwards from microprocessors, these robust old-fashioned electronics would survive a nuclear explosion when our TV’s and cellphones had all fried in the flashover.
But is there any evidence for what Monadhliath actually is? Its existence is alluded to in Duncan Campbell’s "War Plan UK", which predicts the fate of atomic power stations, and coal-fired giants like Drax, during a war. Peter Payne’s "The Hydro" summarises the Mackenzie Report’s findings, going into some detail with Monadhliath 1 and 2, even locating them on its endpaper maps.
Yet the clinching proof is an image taken from an obscure Swiss journal on hydropower*, which confirms the scale of the turbine hall, and that the sets were manufactured for the Swiss-Swedish firm ABB, most likely by a Clydeside shipyard, the only fabricators capable of dealing with the scale of the turbines' high-tensile steel blades and casings.
Monadhliath is more than I've suggested, though. Designed by the Property Services Agency, who were responsible for all government works during that era, there is a clandestine air about Monadhliath which doesn't only arise from its purpose. After all, Ben Alder is one of the remotest tops this side of Knoydart – far out of the way, and bleaker than any other Munro – so who would care what went on there?
Perhaps Monadhliath is still held in strategic reserve. Just don't ask Scottish & Southern Energy, or the Scottish Government, or Westminster, to confirm the rumours about this place. They'll deny its existence, just as they have done for the last 30 or more years.
*Zeitschrift fur Hydro-technik, published by Kirschner Verlag in Berne.
