With some of the world's superpowers recently sending spacecraft headed for Mars, it might be easy to forget that our own Little Blue Dot still has a lot of secrets left to uncover. From the depths of the ocean to the middle of the city, there are certain events that tend to fly over everyone else's heads.
Some of history's mysterious sounds have been solved, as the case with the infamous "Bloop" of the 90s, now known to be caused by Antarctic ice cracking and falling into the ocean, picked up a distance away. Here are four of the strange sounds picked up and no one can still explain.
Julia
In 1999, the US National Oceanic and Atmospheric Administration picked up an eerie sound lasting for approximately 15 seconds. It was likened to scratching or cooing. The unexplained sound, although coming underwater, was loud enough to be picked up by the Equatorial Pacific Ocean autonomous hydrophone array.
A number of theories have been put up about "Julia," the leading of which was an Antarctic iceberg running aground. The sound has been estimated to come from Bransfield Straits and Cape Adare in the Antarctic region.
Skyquakes
Not every sound mankind failed to explain is from the earth or under the ocean. Across the sky, there are spontaneous sounds of explosions, much like a sonic boom. It has been historically reported for at least the last two centuries. Moreover, these loud rumblings from up above are usually recorded in cities or villages near bodies of water - from the US' Finger Lakes and Magic Valley, to India's Marwari village, to Bandung, West Java in Indonesia.
It is important to note that these are different from thunder strikes, mostly because of the absence of clouds to generate lightning. Among the most popular explanations include coronal mass ejections and meteors entering the atmosphere, as well as large natural disasters such as avalanches and focused thunder strikes in other parts of the world.
The Hum
Unlike other entries in this list, this is not a one-time or a geographically-limited incident of a spooky sound leaving people puzzled. Globally known as "The Hum," it is a low-frequency droning sound that has reportedly been heard by a select number of people. Mostly reported in the UK and the US, newspapers have followed and reported instances of the unexplained sound from as far back as 1973.
The event was also heard in other parts of the globe.
Theories about the Hum have ranged from the mechanical (jet streams, diesel engines), to a wave of low-frequency tinnitus, to other elements in the local ecosystem (toads, fishes, to insects).
A 52Hz Whale
Whales are commonly found in pods, traveling across oceans together with their kind. However, since the late 1980s, scientists have been monitoring a whale, all one, emitting a call not found in another incident. Its recorded whale call is pegged at 52 Hertz, with its sad calling creature being called the "52-Hertz Whale" as well as the "world's loneliest whale."
Through its migratory pattern, scientists have narrowed down the species to the blue whale or the fin whale. However, the 52 Hertz whale's call does not fit the frequency of any of the aforementioned species.
For the weirdest things ever found outside our home planet, check out 4 Most Interesting Things Ever Found Across the Universe
