Set to be totally transformed by 2018, the mine will become a 200-megawatt pumped-storage hydroelectric reservoir, which means it will behave as a battery and have the energy to power more than 400,000 homes.

When needed to compensate intermittent wind and solar power, as much as 1 million cubic meters of water could be allowed to plunge as deep as 1,200 metres, turning turbines at the foot of the collieries mine shafts. The mining complex comprises 26 kilometres of horizontal shafts.

Miners in the town of Bottrop, who have worked for decades at the site, will remain employed while seeing a shift in their usual tasks. According to governor Hannelore Kraft, they will continue playing a key role in providing uninterrupted power for the country.

During a press briefing earlier this week, Kraft also said that other mines may follow suit because the state needs more industrial-scale storage as it seeks to double the share of renewables in its power portfolio to 30% by 2025.