Hidden away above the tiny Swiss Alpine town of Linthal, deep inside a snowcapped granite massif, sits Europe’s newest engineering marvel. It is a hydropower plant like no other, able to generate as much electricity as a nuclear power plant and, at the flip of a switch, act as a giant battery. “It’s the only grid-scale method of storing energy,” says Maryse François, the hydrotechnology leader at GE Renewable Energy, the company that developed the technology powering the site.