In the spring of 1972, President Richard Nixon signed off on Operation Pocket Money, a plan to deploy 11,000 sea mines off the coast of North Vietnam to cut off naval supply routes to the region. These underwater explosives were rigged to detonate in response to magnetic, acoustic, and pressure signatures from passing ships.

But on August 4 of that year, a mysterious force triggered a mass explosion of dozens of mines. The likely culprit? The Sun, according to new research published in Space Weather.

Led by Delores Knipp, a space weather expert at the University of Colorado at Boulder, the study suggests that an unusually intense solar storm activated magnetic sensors in the undersea destructor mines, causing them to blow up en masse.

“The extreme space weather events of early August 1972 had significant impact on the US Navy, which have not been widely reported,” Knipp and her colleagues said in the paper. “These effects, long buried in the Vietnam War archives, add credence to the severity of the storm: a nearly instantaneous, unintended detonation of dozens of sea mines south of Hai Phong, North Vietnam on 4 August 1972.”

The study builds on contemporaneous reports from officers who oversaw Operation Pocket Money, some of which are preserved in US Navy documents that were declassified in the 1970s. These include eyewitness accounts of the explosions from US aircraft crew off the coast of Hon La, along with estimates that thousands of sea mines were potentially scrambled or detonated by the burst of solar activity in early August.

The storm was ignited after the Sun belched out energetic solar flares and coronal mass ejections, which sent blasts of radiation, plasma, and charged particles throughout the solar system. The event is apparently legendary at NASA because it occurred between the Apollo 16 and 17 missions, and underscored the threat of space weather for astronauts and spacecraft.

Read More: Humans Accidentally Made a Space Cocoon For Ourselves Out of Radio Waves

But as demonstrated by the explosions of the sea mines, solar activity can also influence machinery and electronics on Earth. Knipp and her co-authors detail the unusual scientific readings, power outages, and widely observed auroral activity caused by this severe storm. They concluded that the 1972 event could have been in the same league as the Carrington Event, the largest geomagnetic storm on record, which occurred in 1859.

“In our view this storm deserves a scientific revisit as a grand challenge for the space weather community, as it provides space‐age terrestrial observations of what was likely a Carrington‐class storm,” the authors said.

Reconstructing the fallout of the storm can help humanity prepare for future Carrington-class tempests. Given how much more we rely on electronic infrastructure on Earth and in space compared to the 1970s, such a storm would have the potential to wipe out power and communications on a massive scale, and create unpredictable havoc similar to the 1972 mine detonations.