The subterranean hydro power station has become known as ‘Hollow Mountain’.
The Scottish government has given the green light to expand a hydro storage plant in the west of the country.
Renewable power developer Drax wants to build a new £500 million (€581 million) development in their existing Cruachan facility.
Authorities have approved the plans, which will also help Scotland in its bid to reach net-zero targets.
But the power company says UK government policy needs to change before construction begins to make the project attractive to investors.
A giant water battery in Scotland
Located in the area of Argyll, Drax’s underground power station is housed in a vast cavern excavated from inside the mountain Ben Cruachan.
The subterranean station has become known as ‘Hollow Mountain’.
The Cruachan plant was inaugurated in 1965 becoming the first large-scale reversible turbine storage energy scheme of its kind around the globe.
Turbines pump water from the lower pool to the upper to ‘charge’ the battery and store energy.
When electricity is needed, the water is released and the flow rotates a turbine which generates hydroelectric power.
A £500M expansion project is the first in a generation
Cruachan plant owner Drax now plans to carry out a 600-megawatt expansion, for which it has received government approval.
"This is a major milestone in Drax's plans to build Britain's first new pumped storage hydro plant in a generation," says chief executive Will Gardiner.
He added that, given the right support from the state, investments would see the generating capacity of the station more than double.
Huge investments are needed for an expansion, which is expensive to build and takes between five and eight years to complete.
Scotland aims to become net-zero
While visiting Cruachan, First Minister Humza Yousaf said the expansion scheme was an important step in helping the country meet net-zero targets.
"The Scottish government will continue to urge the UK government to provide an appropriate market mechanism for hydropower and other long-duration energy storage technologies, to ensure that the potential for hydropower is fully realised," he said.
The British government says it has already secured billions of pounds worth of green investment.
"Our plans to power up Britain are expected to attract a further £100bn investment and support 480,000 jobs across the UK, including Scotland, by 2030,” a spokesperson told the BBC.
"Pumped hydro storage will help deliver greater energy security and economic growth and we have already confirmed our intention to enable investment in these technologies while removing regulatory barriers."
Hydropower will ‘ensure energy security’ for Scotland
Trade body Scottish Renewables said the expansion to the hydro plant and other projects of its kind would be integral to ensuring energy security and lowering energy bills.