Infrasound

Infrasound includes all sounds below roughly 20 Hertz. Such frequencies are too low to be heard directly by the human ear except at volumes much louder than frequencies above 20 Hz, 70-100 dB(G) depending on frequency. They are produced during extreme weather, earthquakes, explosive detonations, meteor impacts, turbulent airflow over mountains, the collision of oceanic waves, and other large scale releases of energy, but they are also produced by waves on beaches, the wind and air conditioners in low sound pressures, and by elephants and giraffes.

People cannot consciously hear it, but it has been shown to cause strange and uneasy feelings when played to a crowd at very high decibel levels, which may explain certain paranormal activities. For this reason, it has been occasionally advocated as a non-lethal weapon for riot control. It can be felt directly through the chest and stomach and may affect heart and digestive function, leading to the myth of a and the potential for sonic weapons that could kill or injure. This potential was explored by Nazi engineers but never actually utilized.

The ability of certain animals to hear this type of sound explains why they go crazy and flee inland as a tsunami approaches, scaring the others and causing them to follow. This is good to know, because the less-learned of creationist types will use this anecdote as proof that animals can be guided by the hand of God, which is why some Babylonian myth about a boat is true, and why the geological column is sorted the way it is, and other such nonsense. Some animals, especially birds, might also be able to detect the electromagnetic disturbance caused by the shifting of underwater rock that creates the tsunami.

It is said Infrasound also makes people spiritual. Additionally, it may account for some ghost sightings, reports that a place feels "haunted", or that someone unseen is watching them.

Wind Turbine Syndrome
Some people who live near man-made sources of infrasound, including older wind turbines and industrial ventilation fans, have claimed to suffer ongoing headaches, tachycardia, and unexplained fear and emotional distress. This has become known as wind turbine syndrome. There is no physiological basis for this claim, or for Wind Turbine Syndrome.

Nuclear detonations
Infrasound is released by nuclear explosions and monitored by international authorities in order to enforce the Comprehensive Nuclear Test-Ban Treaty.