Between 250,000 and 400,000 cubic metres of rock could break off the Hochvogel peak in the Alps, on the border between Germany and Austria, researchers at the Technical University of Munich have warned. They raised the alert level after data from the early warning system.
Hochvogel is 2,592 metres high and stands in the Allgäu Alps, on the border between Germany and the Austrian state of Tyrol. In June, about 2,000 cubic metres of rock fell from the mountain.
Measurements in spring showed the rock is shifting by up to 5mm a week. Geologists see that as very fast: in one week the peak moves more than the Alps do on average in a whole year, about 1.8mm.
Scientists fear the summit itself could split away from the massif. The cause is a crack that has existed since the 19th century and keeps widening. It was once as wide as a person's shoulders. Now it is several metres wide.
Last year the team gave its size: up to 6 metres wide, about 35 metres long and about 60 metres deep. It is one of the peak's weak points, along with a new crack on the south-west ridge, which the scientists have named "Crack 6".
Researchers have tracked the movement of the rock for 12 years. More than 30 sensors across the peak measure shifts non-stop, to an accuracy of 1mm, and the data arrives several times an hour. A drone with a laser scanner has also scanned the peak.
The scientists say the rock is also moving faster because of extreme weather: heavy rain, storms and melting snow. Johannes Leinauer of the research team said storms are becoming more frequent and stronger because of climate change, and this may affect how often rockfalls happen and how fast they are.
The Austrian side of the mountain has been closed to hikers for some time, while climbing from the German side is still allowed.