Slope failures often happen after rain, but we don't fully understand why. Scientists did tests on model slopes with sand to figure out how rain causes slope failures. They found that failures happen when the soil at the bottom of the slope gets really wet, even if the rest of the soil is still somewhat dry. The slope also starts to move a little before it fails. By watching how wet the soil is and measuring how the slope moves, we can predict when a slope might fail after rain.