On a hot day in August 1972 toward the end of the Vietnam War, dozens of naval mines off the coast of Hai Phong in North Vietnam began to explode without warning. In March 1989, a magnetic surge tripped circuits, knocking out power in the entire Canadian province of Quebec. While in 1859, an event sparked telegraph lines, igniting fires, and northern lights so bright that British stargazers could read newspapers at night. These days, scientists know that all these events were caused by intense space weather, capable of wreaking havoc on electric grids and electromagnetically sensitive technology.