The original version of this story appeared in Quanta Magazine. For most of humankind’s existence, Earth was the only known ocean-draped world, seemingly unlike any other cosmic isle. But in 1979, NASA’s two Voyager spacecraft flew by Jupiter. Its moon Europa, a frozen realm, was decorated with grooves and fractures—hints …

Late last week, on Iceland’s Reykjanes Peninsula, a concerning sequence of earthquakes suddenly turned into a full-blown volcanic crisis. A burst of intense and frequent seismic shaking, accompanied by a convulsing crust, suggested that a huge volume of magma was rapidly burrowing its way toward Svartsengi, the site of a …