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Episode 03

How Do the Swiss Know When a Glacier Will Fall?

250,000 tonnes of ice hangs above Randa, sliding up to 20 cm daily. Nobody can name the day it falls, but the Swiss watch it speed up and a radar shuts the road.

27 October 20266 min readVideo 8:48

The Swiss can't name the day a hanging glacier will fall. What they can do is watch it speed up along a curve that points at its breaking time, listen to it crack, and let a radar close the road and railway the moment it falls. Above the Swiss village of Randa, a camera and a radar are already on the job.

Watch the episode: This Glacier Slides Up to 20 cm a Day Above a Village 8:48

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The ice above Randa

About 250,000 tonnes of ice on the Weisshorn's east hanging glacier could break off above Randa. The Weisshorn is 4,505 m high, and the ice is sliding downhill by up to 20 cm a day, according to Norbert Carlen, a natural hazards engineer for the canton of Valais. At that top speed, it adds up to 1.4 m a week. Ice isn't supposed to be in a hurry.

For scale, 250,000 tonnes is about the weight of 37,000 big bull African elephants, at about 6.8 tonnes each.

Randa has had the glacier watched more closely since the end of June 2026. On 24 August, after movement increased clearly, monitoring was raised to level 3 of 4, and at least three ice break-offs were recorded that day. No one may stay in the marked danger zone.

The village has 479 residents, and its mayor says the residential area is not in the danger zone, that the glacier has been monitored for years, and that there are always movements. Martin Funk, an ETH Zurich glaciologist who co-wrote the study of the Weisshorn's 2005 break-off, called the developments typical behaviour for hanging glaciers and nothing new.

So I wanted to know: can anyone tell when a glacier will fall?

Why does a glacier hang off a mountain?

A hanging glacier's front grows by ice flow and snow accumulation until it fractures. Glaciologists define hanging glaciers as unbalanced: they must shed ice regularly just to keep a finite size. Falling off is part of the job.

What holds one up is how much of its ice is frozen to the bed. It can let go in two ways. The front grows until it snaps off, or, in temperate ice, meltwater changes how the base slides, the drag drops, and a whole slab breaks away.

Before a collapse, only its effects can be detected: changes in shape, new crevasses and a rise in speed.

Above Randa, ice of various sizes breaks off often from the lower part of the glacier, where the slope is steeper than 60% in places.

Has this glacier fallen before?

It has:

1636: in January, a disaster in Randa killed 37 people.
1720: 84 years later, 12 people died in a very similar event.
1819: the largest recorded event. On 27 December, an ice and snow avalanche destroyed 113 houses and killed 2 people.
1972 and 1973: smaller break-offs stopped on a glacier field before reaching inhabited areas.
1991: not ice but rock. In April and May, two rockslides released almost 30 million cubic metres, about 9,600 Olympic pools, and buried the rail and road links to Zermatt. No one died.

Weisshorn avalanches have killed 51 people since the 17th century.

Switzerland has also seen what happens when nobody watches. On 30 August 1965, 2 million cubic metres of ice and debris from the Allalin glacier engulfed the Mattmark dam construction site and buried 88 workers. The glacier was never monitored, and no warning system was installed, despite experts' recommendations. The workers' dormitories had been built in its path.

How do you watch a glacier?

Mostly, you photograph it. In late October 2018, a camera was installed at 4,133 m on a peak of the Bishorn, facing the Weisshorn's northeast face. It photographs the glacier several times a day. Software compares small fields of the images to measure how far the ice has moved.

The first attempt to predict such an icefall was made in 1973, on the Weisshorn, by measuring how fast the unstable glacier's surface moved. Before a break-off, the surface speeds up following a power law that runs to infinity at a finite time: the time of failure. In theory, you can read the moment off the curve.

The Weisshorn tested it again in 2005, when its hanging glacier broke off in two steps, on 24 March and 31 March. About 500,000 cubic metres of ice had been unstable, about 160 Olympic pools.

Researchers measured surface motion and icequakes for 21 days, up to 3 days before the break-off. A single geophone was buried in the firn about 30 cm below the surface, near the upper crevasse. The icequakes increased before the fall, and the seismic activity sped up after 18 days.

Fitted afterwards to the 2005 data, the model put the critical time within half a day of the break-off. A later model, combining motion and icequakes, could detect a rupture 15 days in advance. The catch: it was a retrospective analysis, and real-time use needs automatic seismic data processing.

Does all this watching save anyone?

In May 2025, above the village of Blatten, falls from the mountain dumped about 3 million cubic metres of rock onto the Birch Glacier in a fortnight. Because of the high risk of rockfall, around 300 people were evacuated.

On 28 May 2025 the glacier collapsed. Over 9 million cubic metres of rock, mud, ice and debris came down and engulfed the village in about 40 seconds, destroying most of it. All 300 residents had been evacuated in time. A 64-year-old local shepherd died.

The Birch Glacier had been monitored since 1993. Still, glaciologist Christophe Lambiel says surprises must be expected, and where and when is very difficult to know. Valais has geologists, 90 local observers, numerous monitoring devices and early-warning tools, and Switzerland cannot monitor every peak.

What happens the second it falls?

For that moment, Randa has an avalanche radar watching over the cantonal road to Zermatt and the Matterhorn Gotthard Railway. It detects falling ice up to 5 km away, about 50 football pitches, around the clock, through fog and snow and at night.

It ran as a test in the winter of 2017 to 2018. On 20 January 2018 at 9 pm, an avalanche reached the cantonal road but did no damage. At test sensitivity, the radar detected more than 700 events in one year. From the 2018/19 winter it was connected to the signalling, so when a major avalanche comes down, the road and railway close automatically, and reopen afterwards.

Nobody knows how this one ends. The canton's natural hazards head sees two scenarios: the glacier keeps disintegrating gradually, or the whole unstable mass breaks away at once. For now, there is no indication an ice avalanche could reach the village.

That ice up there is moving

So, can the Swiss tell when a glacier will fall? Not to the day. They watch it speed up for months, and when it goes, the road and railway close themselves.

The ice you see from a valley floor is moving. In some valleys, a camera is measuring exactly how fast. Is there something near you that everyone watches: a river, a cliff, a bridge?

Sources

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