From mysterious skyquakes to enigmatic undersea shrieks, Lauren Fuge finds five sounds that have defied scientific explanation.

A lot of unexplained sounds – some one-offs, some repeating – have been reported around the world.

Some have remained mysteries for years before finally being solved, such as the ‘Bloop’, an extremely powerful, ultra-low-frequency sound so loud that not even a blue whale could belt it out. A frenzy of speculation followed its 1997 discovery by deep sea microphones. Was there some even larger animal lurking in the unexplored deep, bigger than anything we’ve ever known? No, it turns out: 15 years later, scientists declared that the Bloop was caused by an enormous ice shelf cracking apart in the Antarctic.

The Bloop is not the only sound to stir up excitement with thoughts of the unknown; many sounds still have scientists scratching their heads. Let’s take a look at five.

1. The Hum

One explanation for the Hum is Very Low Frequency (VLF) radio transmissions. Alengo / Getty

A constant, low-frequency sound has been plaguing people around the world since at least the 1960s, from Canada to New Mexico, Scotland to New Zealand. Most who hear it say it sounds like a truck engine idling and earplugs don’t help to block it out. Called the Hum, it is so well documented that we even have some stats: it can only be heard by about 2% of the population, it is generally present indoors, it becomes louder at night, it is heard more often in rural and suburban areas, and it tends to be heard by middle-aged people.

Some doubt the Hum is even a physical sound; in a fraction of cases it may be the result of psychology, with people focusing too hard on ambient noise. But for most, the Hum appears to be very real.

Many obvious sources have been ruled out, such as highway noise, industrial equipment, the electricity grid and phone towers. Other theories of varying plausibility have been suggested, such as earth tremors, mating fish, power or gas lines, tunnelling under the Earth, wireless communication devices, and the obligatory aliens.

A study by geoscientist David Deming, of the University of Nebraska, suggests the Hum may actually be a result of Very Low Frequency (VLF) radio transmissions used by military powers. Other research suggests the Hum comes from natural terrestrial or geological phenomena. It is a well-studied fact that animals seem to be able to predict earthquakes, so perhaps some humans have the same mechanism.

2. Skyquakes

Skyquakes are mysterious booms from the sky – but are not thunder. Valery Hache / Getty Images

Skyquakes are another sound heard around the world. From the River Ganges in India to the Sea of Japan, these mysterious booms sound like cannon fire rumbling down from the sky. They are commonly heard near water, occasionally rattling windows and plates. Some have been explained by military aircraft breaking the sound barrier, but that doesn’t account for reports of skyquakes heard as far back as 1824.

Scientists have come up with a few likely causes. Near coasts, the booms may be caused by enormous waves crashing against the cliffs. Sand dunes are also capable, through unexplained mechanisms, of producing sounds including, on rare occasions, large booms. Other options are meteors generating sonic booms as they speed into the atmosphere; shock waves caused by coronal mass ejections from the Sun smashing into the Earth’s magnetic field; distant volcano eruptions; far-off thunder redirected through the upper atmosphere; deep earthquakes making noise by cracking the crust; and gas belching up from underground vents beneath ocean or lake beds.

As with the Hum, it is likely that a combination of these explanations are the culprits.

3. The 52-Hertz whale

This mysterious call may come from a hybrid between a fin whale and a blue whale, pictured. Science Photo Library / Getty

Now for a sound that may break your heart: a distinctive whale call that is different to any other whale we know. The sound has been tracked in the North Pacific by a classified array of navy hydrophones since 1992, but has never been seen or studied close-up. Intriguingly, its movements appear to be unrelated to those of other whales, though they share certain similarities with blue and fin whales. The short, frequent calls are at an unusual 52 Hz, much higher than other whale species; blue whales usually call between 10–39 Hz and fin whales at 20 Hz.

To explain this strange song, scientists speculate it comes from an animal that may be malformed or a hybrid such as the offspring of a blue whale and a fin whale. Others simply suggest it may sing in a dialect. Though the sound has been dubbed as the call of “the loneliest whale in the world”, whale vocalisation experts are quick to point out it has many of the same features of a typical blue whale song, so other whales can definitely hear it – and possibly even respond to it. This whale is just a bit of an oddball.

4. The Upsweep

The Upsweep may be caused by undersea volcanoes. Bob Halstead / Getty Images

The Upsweep is composed of a long train of sounds that sweeps repeatedly upwards like an unearthly howl, from low to high frequency. It was first detected in the Pacific ocean in 1991 by the autonomous hydrophone arrays of the US National Oceanic and Atmospheric Administration (NOAA).

Interestingly, it changes throughout the year, peaking in spring and autumn, although scientists are unsure whether this is due to changes in the source or changes in the environment the sound travels through.

The Upsweep has a plausible but unconfirmed explanation: undersea volcanoes. It is thought the sound may result from hot lava pouring out into cold seawater. Though it can still be detected, the level of the Upsweep has been slowly declining since its initial discovery.

5. Colossi of Memnon

The Colossi of Memnon on the West Bank of Luxor are 17 metres high and cut from two massive granite blocks. Paul Panayiotou / Getty Images

Many of these sounds have been heard for decades or even centuries, but let’s go back millennia to the strange case of the singing Egyptian monument.

West of the River Nile near Luxor, Egypt, two massive twin stone statues stand proudly. Called the Colossi of Memnon, they are a tribute to Pharaoh Amenhotep III. In 27 BC a large earthquake shattered part of one of the colossal statues, cracking the lower section and collapsing the top. Soon people began to notice something strange – the statue started to ‘sing’. The sound usually came at dawn and was primarily reported in February or March, though that probably indicates the tourist season rather than a real trend, as the mysterious sound drew people from all over. To Greek historian and geographer Strabo it sounded like a blow, while Greek traveller and geographer Pausanias compared it to the string of a lyre breaking.

Scientists today speculate the sound was caused by a rise in heat and humidity in the ruins of the stone as the Sun rose. But they can’t check their theory, because although the statues are still around, the sound is not. In about 199 CE, Roman emperor Septimius Severus ordered the repair of the earthquake damage – and the singing disappeared.