Noise Addict has some Earth-y tones that anything but obvious in their origin.
The Bloop
At various times during the summer of 1997, an ultra-low frequency sound that rose rapidly in frequency over about one minute was detected at 50 degrees S, 100 degrees W. The sound was detected by the US National Oceanic and Atmospheric Administration with the Equatorial Pacific Ocean autonomous hydrophone array (which was U.S. Navy equipment originally designed to detect Soviet submarines), and was 
loud enough to be heard on multiple sensors, up to 5000km apart. Scientists dubbed it the "Bloop" (not to be confused with the "Boing".)
Although the sound matches the profile of a living animal, it is much louder than any known creature can produce. Any creature that could produce such a sound would have to be many times larger than the largest whale. You can hear a very short recording of the sound here . The recording is short because it’s been sped up 16x to make it audible to you and I. Some people link the Bloop to Cthulhu , a mythical creature from an H.P. Lovecraft story as the noise originated from an area near the mythical sunken city of R’lyeh from the same story. The Bloop also makes an appearance in the game promoting the movie Cloverfield, and was also seen in the movie "The Loch", coming from a giant eel. A 2001 album by Dntel ("Life is full of possibilities ") uses the bloop as a repeating sample through the piece. The actual origination of the sound is not known and remains a mystery to this day although it is suspected to be biological in origin.
The Hum
The hum is the name of a phenomenon that is generally given to mysterious low frequency humming or rumbling. It is typically heard by many people at a time (but not others), and can come and go or it can be constant. There are many famous Hums, most notably the Taos Hum and the Bristol Hum.
The Hum is usually difficult to record, and it’s often difficult to localize the source of the hum (perhaps due to the low frequency, as low frequency sounds are harder to localize).
Hums have been detected (or reported) all over the world, but most appear in Europe and South America. The Hum is more often heard indoors, and some people hear it more faintly than others. Here is a recording of the Auckland Hum.
The Taos Hum has been featured on the X-Files and Unsolved Mysteries. The source of some Hums have been identified - for example, a pair of fans in a cooling tower at a DaimlerChrysler casting plant was emitting a 36 Hz tone that caused a Hum over the entire city of Kokomo, Indiana. Other Hums remain a mystery. Some possible explanations Include geological events, pulsed microwaves and electromagnetic waves from meteors. Tinnitus might explain some cases as well.
A creditable scientific hypothesis from 2005 suggests the Hum is caused by the tensor tympani muscle (a muscle in the inner ear) trembling in the eardrums of individuals. on the eardrums of affected individuals by the tensor tympani muscle trembling. There is a website by the "Interest Group for Research of the Hum Nuisance" (unfortunately in German) describing this theory.
The Hell Hole
More than forty years ago, researchers in the Soviet Union began an ambitious drilling project whose goal was to penetrate the Earth’s upper crust and sample the warm, mysterious area where the crust and mantle intermingle– the Mohorovičić discontinuity, or "Moho." This type of drilling was completely new and the technology didn’t exist to go that deep, and so the Russians had to invent a completely new was of drilling to be able to do it. Unfortunately, the Russians never reached their goal, and many of the Earth’s secrets were left undiscovered, however The Kola Superdeep Borehole is still a scientifically useful site, and research there is ongoing.
When drilling stopped in 1994, the hole was over seven miles deep, making it by 
far the deepest hole ever drilled by humans. The last of the cores to be plucked from from the borehole were dated to be about 2.7 billion years old. Although the Kona hole was the deepest hole ever drilled (until this one), seven miles was still very short of the 20-80km required to penetrate the earth’s crust.
Like all newfangled science stories, some Genesis freaks have decided that the intent of the project was not real scientific research as they were told - rather this simple experiment was actually an attempt to drill to hell… and that they were successful! The story has (and still does) made its rounds on Christian circles via tracts, preaching and radio broadcasts.
The story varies, but here are the basics:
- After going only a few miles down, the drill began to spin wildly.
- A ‘Doctor Azzakov’ is quoted as stating authoritatively that it has been shown that the earth is hollow.
- Immensely high temperatures were experienced, much higher than expected at that depth. Usually 2,000 degrees Fahrenheit or 1,100 degrees Celsius is quoted.
- Microphones were lowered into the hole (to ‘listen to the earth’s movement’). Human screams were heard—hordes of ‘tortured souls’.
- Many of the scientists have quit the project in fear and/or have become total nervous wrecks.
Of course, these "facts" are not quite true: If the earth was largely hollow, it would clearly be evident from seismic studies, as well as from orbital/gravitational considerations, but this is not the case. Far from being a ‘fiery inferno’, the temperature increased by one degree Celsius every 100 meters to 3,000 meters, then by 2.5 degrees every 100 meters thereafter. At 10,000 meters, it was only 180 degrees.
The story of course is based on a factual borehole, and creation geologists have had a field day with the shaky "facts" - using the story to prove that yes, hell exists and they’ve been right all along. Here’s the "quote" that has been making it’s way through evangelical circles:
"We lowered a microphone, designed to detect the sounds of plate movements down the shaft. But instead of plate movements we heard a human voice screaming in pain! At first we thought the sound was coming from our own equipment. But when we made adjustments our worst suspicions were confirmed. The screams weren’t those of a single human, they were the screams of millions of humans!"
Oh, you wanted to HEAR the screams from hell? But of course! Listen to it here (or here ) .
Mistpouffers
In some places in the world, people have reported long successions of enormously loud booming noises. They are called different things in different areas of the world - “Guns of the Seneca” (near Seneca Lake in New York), “Barisal guns ” (in Bangladesh), “uminari” (in Japan), “fog guns,” “lake guns,” and many other terms. These terms all describe a sound or sounds that resemble distant cannon fire, and are usually heard near large bodies of water. Often times they are accompanied by a long rumble that is strong enough to shake plates and pictures.
There have been many proposed theories about where these sounds come from, however most are not very satisfying. Since these sounds have been reported for centuries means that the most obvious explanation, artillery tests , are pretty much ruled out. Earthquakes and volcanoes could produce these sounds and rumbles, however the sounds have not been directly connected to any seismic activity, which is fairly well measured. Some have speculated that undersea activity (perhaps seismic) creates great bubbles of released gas which floats to the surface and creates huge "ocean farts", however it is a stretch to think that these bubbles could produce a sound strong enough to create the distant-gunfire sound of Mistpouffers. Meteorite impacts have also been bandied about as a possible explanation (see here for actual meteor sounds) as have tidal waves . 
It has also been speculated that these noises happen everywhere and that ambient noise from communities simply make them harder to hear. Sound travels farther over water than over land, and so the sounds are more easily heard in remote, quiet areas close to bodies of water.
Of course the latest theory is rather boring - that the sounds are made by thunder or other explosions very far away, and the sounds simply travels a very, very long way because atmospheric and topographic conditions happen to be "just so". This would explain why no storms or other activity are present in the area and yet the sounds are still heard.
Some people still believe that the sounds are made from alien spacecraft, God, or Thor’s hammer banging on nails while trying to fix the roof over the heavens…. however there is another theory: A web page describing the many tourist attractions of the Cayuga Lake area mentions the “Guns of the Seneca.” it also says “At the southern end, you’ll find the booming city of Ithaca…” Well, that it. What people are hearing is obviously the sound of Ithaca booming.
Slow Down
Slow down was recorded in the Pacific Ocean on May 19,1997. It was 
recorded by the U.S. National Oceanic and Atmospheric Administration located around 15 degrees S 115, degrees W. It is called the Slow Down because the sound slowly decreases in frequency over the span of about 7 minutes. It was detected using the same hydrophone array as the Bloop, and was loud enough to be detected on multiple sensors 2000km apart. Here is a recording of the sound, sped up by 16 times. Some people believe that this sound has been made by a giant squid or other large sea creature, however this theory doesn’t stand up to scientific reason, as squids likey with tdo not have the capability of producing their beaks ese sounds. The real source of the Slow Down sound remains completely unknown. This signal and anything like it has not been heard before or since.
The WOW!
No discussion of mysterious sounds would be complete without this one, although it’s not a sound from earth - it’s 
from space. On August 15, 1977 a SETI scientist working at the Big Ear radio telescope of the Ohio State University noticed a very strong signal that lasted for 72 seconds. The type of signal resembled signals that are non-terrestrial and non-solar system in origin. Because the signal was so remarkable, The scientists circled the data on the computer printout and wrote the word "WOW!" beside it. Ever since then, it’s been called the "Wow!" signal.
Since the signal was discovered, scientists from all over have tried to locate it again, however it has never been seen since. It has been theorized by some people that the signal may have come from extraterrestrial life, however others remain skeptical. More information on the Wow can be found here by the person who discovered it.
