Watch Thousands of Tons of Mud Surge Down a Swiss Mountain

The Swiss canton of Valais is famous for its picturesque resort towns, craggy mountains, and scenic trails—but come spring, tourists and locals alike should probably tread carefully while hiking near lllhorn Mountain. Located in the Swiss Pennine Alps, the peak is the site of a massive annual mudslide that strikes without warning once warm weather arrives.

lllhorn Mountain sits in Illgraben, a valley that formed from erosion during the 14th century. According to The Washington Post, balmy spring temperatures cause layers of snow and icy dirt to melt, prompting mud and debris to flow down the mountain. As the ooze cascades down, the ground shakes, and the air is filled with a sulfurous aroma.

Scientists use the site to observe debris flow, but some shutterbugs like to capture the natural phenomenon on camera. Twice now, a man named Pierre Zufferey has successfully filmed the Illhorn cascade—and when this year’s mudslide hit on May 29, Zufferey was there to document the deluge in a two-part video.

The 2017 Illhorn mudslide contained 25 Olympic swimming pools' worth of mud and debris, according to Swiss natural research institution WSL. Luckily, residents of Susten, a nearby town that sits downstream, were warned in advance, giving them enough time to steer clear.