Yet another theory points to the nearly 8m lightning strikes that hit Earth every day. Other scientists have suggested vibrations caused by volcanic eruptions and earthquakes might be responsible (the eruption of Krakatoa in 1883 left the Earth reverberating for days). In 2015, a team of French scientists proposed that the Hum was caused by ocean waves extending down to the ocean floor, and shaking the Earth as they collided with ridges and continental shelves. At an Institute of Biology conference in 1973, it was suggested the jet stream shearing against slower-moving air might cause a very low frequency sound which could then be amplified by electricity pylons. Though there must certainly be cases of localised hums caused by overzealous warehouse fans and horny fish, there are too many persistent commonalities between global reports of the Hum for there not to be the lingering question – is there something larger at play here? A number of natural theories have been proposed over the years, all of which have a certain poetic sublimity to them. In the case of Hythe, the Scottish Association for Marine Science hypothesised that the noise might be caused by the mating call of male midshipman fish, which emit ever-louder drones, sometimes for hours, in an escalating competition to attract potential mates. Studies are sometimes conducted, and theories proposed, but without conclusive findings. Numerous reports of the Hum have been made across the UK, usually clustered around specific towns or cities: Hythe, Plymouth and, as recently as last month, Swansea. But according to some Bristolians the Hum persists to this day, despite the warehouse having long been decommissioned. A prevailing theory was that the Bristol Hum originated from large industrial fans used at a warehouse in nearby Avonmouth. What began as an irritating if innocuous noise eventually drove many who heard it to distraction, and was said to be linked to two suicides.
The earliest reliable reports of the Hum date from the UK in the mid-1970s, most notably from Bristol, when letters began appearing in the Bristol Evening Post about a low rumble heard by dozens of residents throughout the city. If you do hear it, you’re among the roughly 4% of the world’s population affected by “the Hum”, a frequently reported but little understood global phenomenon. Maybe it causes you headaches, dizziness, even nosebleeds. You can hear it when you’re outside, but it seems louder indoors, and particularly at night, when you’re lying in bed.
Or the atmospheric din of an airplane flying overhead, that never gets further away. It sounds like a truck, idling on the street in front your house. In fact, you might not have even noticed it yet – but once you do, you can’t stop hearing it. A low frequency hum, almost a vibration, just on the threshold of human hearing.
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