No volcano erupts without warning signs, caused by rising magma triggering earthquake swarms and inflating the ground surface. The problem is that out of our world’s 1,300 or more active and potentially active volcanoes, we monitor only a few hundred. The Tambora eruption reinforces the unofficial volcanological axiom: the longer the wait, the bigger the bang. That rule of thumb is borne out by the fact that fully half of the biggest eruptions since 1800 originated at volcanoes that had previously been dormant throughout history. What we should be keeping a special watch on then, in order to prepare ourselves for the next arrival of Vulcan’s shock troops, are those seemingly innocuous volcanoes that have kept their heads down for centuries or even millennia. While there are too many candidates to keep a serious eye on, the numbers can be narrowed down by focusing on those that have been recently “restless”; perhaps best regarded as the volcanologists’ term for “bubbling under”. Beyond that, though, it’s anyone’s guess.