In 1972, the United States was deeply involved in the Vietnam War and there was little end in sight. North Vietnam had just launched an offensive in the south, the Easter offensive. The US military was desperate to gain every possible advantage, and Top Brass clad a plan to cover Haiphong harbor with underwater mines.
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As of May this year, Operation Pocket Money dropped thousands of mines off Haiphong Harbor. These mines should have been there for about a year, but on August 4, dozens of them exploded prematurely. But they were not driven off by passing ships. Instead, the mines seem to have been triggered by the sun.
Back then, the suspicion that a military suspected sun disturbance was the explosion, but research has been classified so far. Since the release, a group of civilian researchers have resumed the incident and confirmed the suspicion of the military: Sun effects were to blame.
The key is how the mines are exploded. Each mine has a magnetic sensor that can detect subtle changes in magnetic fields. If a passing ship with its metal shell drifts too close to the mines, the changed magnetic field would trigger the detonator.
Unfortunately, there are many ways to change a magnetic field, except for the hulls. An important source of magnetic fields is the sun, which produces the strongest magnetic field in the solar system. Occasionally, large eruptions from the sun's surface – so-called solar flares – can shoot huge swaths of magnetic material toward the earth.
When these solar flares reach Earth, they can cause all kinds of magnetic disturbances. In their mildest form they are responsible for the Northern Lights and other Auroras. In the worst case, they can bother with GPS systems that interfere with communication and, in a particularly notable case, start almost a nuclear war.
In this case, an unusually strong solar flare was enough to confuse the sensitive sensors of some naval mines in Haiphong Harbor. According to the research paper, the 1972 solar flare was one of the strongest ever recorded, and in addition to the explosion of a few dozen mines, telephone lines were disrupted and blackouts were triggered around the world.
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This event underscores how annoying and dangerous solar flares can be. A high-intensity solar flare, like the 1972 event, could paralyze our satellite networks if it hit us today, and so far we've been fortunate to avoid it. But we can not be lucky forever, and when a 1972 flare reiteration happens, exploding mines are the least of our worries.