On August 4, 1972, dozens of mines seemed to spontaneously explode off the waters of Hon La, Vietnam. The weapons had been planted there as part of Operation Pocket Money, a U.S. plan to block North Vietnam from maritime trade during the Vietnam War, and they were supposed to detonate in the presence of ships. But on that summer day in 1972, U.S. troops flying overhead did not see any vessels that might have caused the mines to go off.

As Becky Ferreira reports for Motherboard, a new study accepted to the journal Space Weather has put forth a possible solution to this mysterious wartime event. The mines, according to the researchers, were likely triggered by a powerful solar storm, which triggered the mines’ magnetic sensors and led to unexpected explosions.

The new research is based in part on declassified Navy documents, “long buried in the Vietnam War archives,” according to the study authors. Navy officials immediately launched an investigation into the unexplained detonations, and they soon suspected that solar activity was the culprit.

As Brett Carter explains in the Conversation, many of the mines that seemed to randomly go off were “magnetic influence sea mines,” which are designed to detect changes in magnetic field caused by passing ships. By the 1970s, it was well known that solar activity could disrupt the magnetic field here on Earth, but Navy officials wanted to confirm that solar activity could also trigger deeply submerged mines. They consulted with experts at the Space Environment Laboratory at the National Oceanographic and Atmospheric Administration (NOAA), and concluded with a “high degree of probability” that the mines had been set off by an intense solar storm.

The new study, led by Delores Knipp of the University of Colorado, affirms this assessment. In the days leading up to the explosions, the researchers explain, a sunspot region known as MR 11976 spewed out “a series of brilliant flares, energetic particle enhancements and Earth-directed ejecta.” A “coronal mass ejection,” or a huge expulsion of plasma and magnetic field from the Sun, reached the Earth in just 14.6 hours; typically, according to Gizmodo’s George Dvorsky, it would take such an event a day or two to hit the Earth’s geomagnetic field. The researchers attribute this speed to two earlier impulses, which “cleared the interplanetary path” for an ultra-fast ejection.

North Vietnam was not the only region affected by this solar storm. Scientists in several locations, including the Philippines, Brazil and Japan, also noticed magnetic disturbances in the atmosphere. On August 4 and 5, 1972, American and Canadian power companies reported power disruptions that ranged from minor to severe, and there were telephone and telegraph outages on a cable connecting Illinois and Iowa.

The researchers say that the event of 1972 was likely “Carrington-class,” referring to a huge solar storm that took place in 1859. During the Carrington Event, which is named after Richard Carrington, the British astronomer who first realized that solar activity could cause geomagnetic disruptions on Earth, “northern lights were reported as far south as Cuba and Honolulu, while southern lights were seen as far north as Santiago, Chile,” writes Richard A. Lovett of National Geographic. In the United States, sparks burst out of telegraph equipment, sometimes starting fires.

Should such an event happen today—when our lives our so intricately linked to technology—the results could be catastrophic, causing mass power losses and disruptions to GPS and satellite communications. So, using modern modelling to better understand solar storms, like the one of 1972, could help us prepare for similar events in the future.

“In our view,” the study authors conclude, “this storm deserves a scientific revisit as a grand challenge for the space weather community.”