Tens of thousands of people can be killed by a single earthquake. So, it’s no wonder humankind has spent millennia searching for an early warning sign. Japanese geophysicist Kosuke Heki recently presented tantalizing evidence that we may someday be able to anticipate these devastating seismic events. Science writer Erik Vance breaks down Heki’s theory that clumps of electrons form in the ionosphere, sometimes 30 minutes or more before a quake.