Each spring, after layers of snow and ice-filled dirt melt under sun and rain, this powerful mudslide comes careening down Switzerland’s Illhorn mountain. It’s predictable only in the sense that it’s annual — other than that, there’s nothing to signal when this flood of dirt and debris will occur.

Pierre Zufferey tried to capture the moment for years unsuccessfully. Last year, he was in the right place at the right time and just happened to catch one of the largest Illhorn flows in recent memory. This year he nailed it again and caught the flow on May 29.

David Petley, a geologist at the University of Sheffield, calls the annual flow “terrifying,” with a “boulder-choked” onset.

Zufferey said the ground trembles during the mudslide and the air reeks of mud and sulfur, which lingers for days.

WSL, Switzerland’s forest, snow and landscape research institute, uses the site to study debris flow. It sets up cameras on Illhorn, which are also used to warn the downstream town of Susten. In 2016, the annual flow was so large that it damaged a lot of the WSL’s instrumentation and cameras. The residents were warned in time this year.

Approximately 25 Olympic swimming pools-worth of mud and debris was discharged in the May 29 mudslide, according to the research institute.