The rule of thumb with a landslide is that however far up a mountain it begins, you're safe if you can get twice that distance away. So, if a fall begins a mile up, you need to be two miles away before busting out the popcorn and beers. Except, that's not really true, since plenty of landslides travel far longer distances than that, causing serious harm to people and property nearby. Thanks to Brown University's Brandon Johnson, we know why: it's because the falling rocks hitting the ground can create a set of very specific vibrations. These vibrations reduce the amount of friction in the local area, causing landslides to flow like rivers.