Image caption The project was abandoned during the Troubles

It's the very stuff of gripping adventure stories - stumbling across the access to a huge hidden tunnel that goes deep into a mountainside.

Just like the forgotten one that runs deep under mountains in Northern Ireland. Few people know it exists or what is at the other end.

But unlike the ones in adventure stories, this one ends rather abruptly at a rock face.

Had things been different it would have ended with a huge man-made cavern which would have housed a power station capable of generating more than 200 megawatts of electricity.

Workers threatened

But the project became a victim of the Troubles in Northern Ireland when workmen were threatened and the project was subsequently abandoned.

The site near Camlough village in County Armagh has long since been forgotten and overgrown.

It's possible to walk past the entrance just a few yards away and not spot it. It's now firmly sealed up for safety.

The tunnel, almost a kilometre long, is substantial. Carved out of the local rock, it's big enough to comfortably drive an articulated lorry and trailer down.

This was to be the access tunnel for what could have been a significant engineering project.

If it had been completed, it would have provided a pumped hydro storage scheme. This is a simple and effective way of storing energy and it's a system that is used across the world.

Water from a lake or man-made reservoir is allowed to fall down a long vertical shaft into a turbine below. It spins the turbine generating electricity.

In effect customers in Northern Ireland are still footing the bill for the decision to abandon the Camlough hydro scheme almost 40 years ago

At night when there is spare capacity in the grid, cheap electricity is used to pump the water back uphill into the reservoir ready to be used again the next day.

While it's an expensive system to construct, its relatively cheap to run as it requires no expensive fuel.

When it was being planned, Northern Ireland power stations had to be kept running all night but most of the electricity was wasted.

Power stations usually have to run at a minimum load and must still be ready to meet sudden surges. That is expensive and it's the electricity consumers who ultimately pay for it.

But if the spare electricity is stored by pumping water into a reservoir, then it can be used the next day and that reduces electricity bills.

Add today's wind turbines into the equation (Northern Ireland has neither hydro nor nuclear generation capabilities) when they are spinning at night and the savings increase.

Northern Ireland also wants to exploit some of the strong tidal streams around its coasts. Some of the strongest tides are at night when no one wants to use the electricity. Storing it would be an ideal solution.

Power station abandoned

But the past civil turmoil and current economic turmoil brought the Camlough scheme to a premature conclusion. The land the tunnel is under is now up for sale.

That wasn't the only loss.

A complete hydro electric power station was also manufactured, paid for and shipped to Northern Ireland ready to be installed. But eventually, after lying around for more than two decades, it was sold, probably for scrap.

Pumped hydro storage is common around the world.

It's used to provide electricity when there is an unexpected surge in demand or to meet excess loads on the grid.

These power stations can get to maximum power from a standing start in just a few seconds. For that reason they also are used as an emergency back-up in the case of a complete grid failure.

Consumers in Northern Ireland have some of the highest electricity charges in the UK, partly because electricity generated at night can't be stored.

So, in effect, customers in Northern Ireland are still footing the bill for the decision to abandon the Camlough hydro scheme almost 40 years ago.

All they got for their money is a tunnel that goes nowhere.