The Plagues of Egypt in the book of Exodus are horrible, fantastical events we have yet to witness on Earth: rivers running with blood, hail mixed with fire, the deaths of firstborns. Some plagues, however, we do see from time to time. For modern-day farmers, locusts are very much a real, destructive force. Swarming in groups by the billions, locusts can travel hundreds of miles, stripping vegetation bare as they go. Scientists have never entirely understood the phenomenon, although one study has suggested swarming results from a lot of banging around as a consequence of overcrowding.