French authorities set about draining a vast underground lake beneath the Alpine glacier of Mont Blanc to avert a potentially disatrous flood.

Some 65,000 cubic metres of water have gathered in a cavity, dangerously raising the pressure beneath the mountain, a favourite spot for holidaymakers, officials said.

They estimated that if the water burst out it could flood the nearby valley of Saint-Gervais in half an hour and “nearly 900 families could be affected,” Saint-Gervais-les-Bains mayor Jean-Marc Peillex said. He said teams were setting up a pump on Wednesday to tap some of the water trapped some 75 metres below the surface, “to let out an initial jet of water in the afternoon to avoid the pressure-cooker effect.”

The pump is targeting the main cavity containing 25,000 cubic metres and could also drain other surrounding pockets, he added. Three other pumps will be brought by helicopter to help with the job, due to last until next month.

An eruption from a similar subterranean lake in the glacier in 1892 caused a torrential flood of water, rocks and earth which engulfed the valley and killed 175 people.