Thousands of landslides per year globallyIn 2020, the World Bank estimated the average annual number of significant rainfall-triggered landslides between 1980 and 2018, by country. And flows happen when the slope material gives way and slides down like a liquid. This is after floods causes massive landslides in Sumatra, Indonesia, in 2024 Image: Antara Foto/ReutersWhy do landslides happen? Landslides happen when the materials that make up a slope fail to resist the forces acting down upon them. Research indicates that the simplest way to do this is to counter the very forces that cause landslides and mudslides in the first place.