Anecdotal evidence spanning many centuries has suggested that animals somehow predict earthquakes before they happen. For example, the Roman writer Claudius Aelianus writes about mice, martens, snakes, and insects fleeing the Greek city of Helike 5 days before a massive earthquake and tsunami destroyed the city. Similar reports have come from zoos (although it is often difficult to distinguish between predictive and reactive behavior in post-quake reports); further, residents of Haicheng China reported snakes emerging from their burrows a month before a devastating earthquake. Other stories abound, including deep-sea fish rising to the surface before massive geologic events and pet owners describing strange dog behavior in the hours leading up to a quake. Are these stories more than merely exaggerated anecdotes?

In 2009, scientists fortuitously documented strange animal behavior before an earthquake. A research group observing a colony of toads in L'Aquila, Italy observed a mass exodus just a few days before a quake (the toad population went from 96 to almost 0 in three days). They hypothesize that chemical changes in groundwater may have clued in the toads to the impending quake. But this explanation must be thoroughly tested, and even if it is true it cannot account for similar behavior observed in deep-sea and ground-dwelling animals,

Other potential explanations include magnetic sensing, foreshock sensitivity at low magnitudes, or perhaps chemical or physical detection at a level humans do not generally observe. While the cause remains a mystery, the lesson is clear: if you see snakes emerging from their burrows months early, frogs fleeing their ponds, or zoo animals panicking en masse-- get the heck out of dodge.