Viewed from up here, the world of man appears very small and vulnerable. The Tête-Rousse glacier, hovering between sky and earth at an altitude of 3,200 metres, dominates the scene splendidly. It is a magnificent panorama of infinite horizons, the perfect silence interrupted only by sound of the climbers’ crampons as they start the ascent to Aiguille du Goûter, the normal route up Mont Blanc. Facing us, the Aravis range and the Chablais Alps break up the horizon, while in the valley below, tiny chalets appear to be clinging to the mountainside.

But the serenity is deceptive. In the core of the glacier lies a silent threat that could, without warning, destroy the village of Saint-Gervais below. Trapped under the glacier lies an enormous 65,000 cubic metre pocket of water – the equivalent of 20 Olympic swimming pools – that could burst and surge down on to the village below. “It’s impossible to predict when that might happen,” said Christian Vincent from the Grenoble Laboratory of Glaciology and Geophysical Environment. He is here to carry out a regular temperature check at Tête-Rousse.

The 75 metre-deep glacier covers 8 hectares of a rocky basin. Early this summer, several boreholes were pierced with a high-pressure hot water drill and special sensors introduced on to the bedrock. Using a snow shovel, the scientist clear the markers that show where these were placed and note down the temperatures. “Precise knowledge of a glacier’s temperatures is vital to understanding how these water pockets are formed,” Vincent explains.