Deep in a Swiss mountain, workers have blasted out a cathedral-sized hole for a power plant that will help keep Europe's lights on, but the profit outlook for the 1.9 billion Swiss franc ($2.1 billion) project has darkened since its construction began in 2008. The Nant de Drance plant, and others like it, is being undermined by cheap renewable solar and wind energy, which up-ends its business model of pumping water uphill at night when power prices are low and releasing it to make electricity when prices peak in the daytime - a process known as pumped storage. These pumped storage plants are designed to balance Europe's power supply grids as they can switch on and off at the flick of a switch, unlike nuclear or coal-fired power plants.