Solar flares set off underwater mines in the Vietnam war – and we just figured it out.

Popular Science can reveal the disturbing solution to a minor military mystery with the story of how space weather made a bunch of military mines suddenly explode:

[I]magine the U.S. military’s alarm when, on August 4, 1972, it witnessed about two dozen or so spontaneous explosions in the waters off Hon La in North Vietnam.

Over four-and-a-half decades later, we now know the culprit was the sun. According to findings recently published in the journal Space Weather, a powerful solar storm likely triggered the mines’ magnetic sensors and caused them to explode.

“It was a storm of magnificent proportions,” says Delores Knipp, a space weather researcher at the University of Colorado, Boulder and the lead author of the new paper. “It was a big story back in the day, and continues to be a big story.” The storm occurred in between Apollo missions 16 and 17, but it’s generally accepted that the radiation dose would have incapacitated (if not outright killed) astronauts traveling to and from the moon. In addition, other studies on the solar storm found the resulting geomagnetic current created many different power fluctuations in North America. “It’s been a storm that has been known for different effects in different communities.”

On that day in 1972, over the course of 30 seconds, American troops flying near North Vietnam witnessed between 20 and 25 explosions in the water, along with an additional 25 to 30 mud spots. Those mines use sensors to detect changes in the surrounding magnetic field density, changes that would normally be triggered by a ship passing above.

“It’s our poster child of storms,” says Knipp. “If a storm that bad were to appear again, then we would really have a lot of problems.” Our current world is immensely tethered to communications instruments, electrical grids, and technology that can easily be fried by particularly strong bursts of solar activity.

Which begs the question: how often do storms like these occur? Underwater mines going off randomly is already a scary possibility to bear, but a loss of $40 billion a day to the U.S. economy is simply unthinkable.

There’s no straight answer to that—after all, the entire field of space weather is a work-in-progress toward this goal. For example, breakthroughs in the last decade have helped illustrate how coronal mass ejections can occur in series rather than just as discrete events, which helped the team pinpoint why solar activity might be powerful enough to set off a couple-dozen underwater mines.

But Knipp says a general estimation, based on current knowledge, is that these sorts of solar storms hit Earth about once every 70 years—“often enough that we need to be thinking about what types of technologies are subject to harm in these kinds of environments.”