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Singing sand is the most easily explained of these auditory oddities. Sand makes sounds described as singing, whistling or barking caused by shear stress as someone walks on it or wind passes over dunes, but only under certain conditions.
These are: round sand grains of 0.1–0.5mm in diameter, sand containing silica, and a certain humidity. The “song of dunes” is a low-pitched rumble of up to 105 decibels that can be heard in around 35 desert locations around the world.
Explanations for the hum are not so clear cut. This series of phenomena involves a persistent and invasive low-pitched noise not audible by all people. It is difficult to detect with microphones and its source and nature are hard to pinpoint. Various locations, such as Taos in Mexico, Hawaii’s Big Island and Bristol in England, have become known for their hums.
According to David Baguely, head of audiology at Addenbrooke’s Hospital, around a third of cases have an environmental cause such as a nearby piece of machinery. And the rest are due to people focusing too hard on a background noise, he believes.
Mistpouffers are described as distant but extremely loud thunder and have been heard in waterfront communities around the world. Suggested explanations include gas escaping from vents in the Earth’s surface, earthquakes and the collapse of underwater caves causing air to rise rapidly to the surface.
The world’s oceans are a rich source of sound. The US National Oceanic and Atmospheric Administration has recorded a host of strange low frequency sounds using a system originally designed to track Soviet submarines.
“Bloop” was detected a number of times during 1997, several times louder than any other known biological sound, at a range of over 5,000km. It has been suggested that Bloop is animal in origin, but a more plausible explanation is ice calving in Antarctica.
Other underwater sounds detected at various times have been given the names Slow Down, the Julia and Upsweep.