Authorities have installed a radar system to better monitor a massive area of a fast-moving glacier on the Italian side of the Mont Blanc massif that risks breaking off and striking a tourist valley.
The radar system installed on Thursday provides constant data on movements of even millimetres.
That compares with the satellite photo monitoring already in place, which requires clear skies and between six and 12 days to get comparative images.
The mayor of Courmayeur Stevano Miserocchi has closed a road due to the risk that a 250,000-cubic-metre mass of the Planpincieux glacier could collapse in the Val Ferret, a popular hiking area nearby.
The alarm was sounded after movements of up to 50cm (20in) a day were recorded.
Higher-than-usual summer temperatures had formed a deep fissure.