Engineers are draining an immense lake that has built up under an glacier on Mont Blanc hoping to prevent a repeat of a flood that killed 175 people more than 100 years ago.
Specialists are drilling into the glacier as part of preparations to slowly pump out the 2,275,000 cubic feet of liquid believed trapped beneath the Tete Rousse glacier.
The water is equivalent to about 26 Olympic-sized swimming pools.
Mayor Jean-Marc Peillex from the French town of Saint-Gervais-les-Bains, whose 3,000 inhabitants would be in the direct line of a flood path, said: "The town didn't want to wait for a catastrophe - we are taking action to prevent one."
The glacier is 10,560ft up Mont Blanc, and is on the most popular paths for climbers scaling Western Europe's highest peak.
Scientists warned local authorities about the trapped water in March. Since then, officials have shut part of a tram route used by Mont Blanc climbers and installed an alarm system in case of flooding. If it goes off, residents have between 10 to 30 minutes to evacuate.
Despite the precautions, officials and scientists say the glacier is unpredictable.
In 1892, flood waters burst from the buried lake, killing 175 people. The amount of water currently believed trapped is comparable to that which caused the 19th century flood.
When pumping starts, it will relieve pressure inside the glacier, the mayor said.
"Once they take out the first few dozen cubic meters, that doesn't mean there will be zero risk, but there won't be at all the same level of stress that there is today," he said.