So, the other night I saw Don Hertzfeldt speak about his short films and some of the techniques he uses when crafting every part of the production. Off-handedly he mentioned that he works in samples of Infrasound - sound that exists in a frequency range below what the human ear can audibly hear but, apparently, can still be sensed by the human body. While the existence of this kind of sound isn't really news I was unaware of all the fascinating facts that go along with it. Such as:

• Infrasound is emitted naturally by avalanches, volcanoes and other severe weather phenomena
• Infrasound is used by a number of animals - whales, elephants, rhinoceros, giraffes - to communicate over long distances.
• When felt by humans, the frequencies can (and often do) invoke a feeling of fear, anxiety and a generally heightened emotional state.
• It has been attempted to be deployed in a weaponized state in actual warfare and for crowd control.
• Researcher Vic Tandy claims that these waves may be the cause of people seeing ghosts, as the resonant frequency of the human eye exists in the same range. Though there are skeptics, he still wrote a paper about it called The Ghost in the Machine.

There's more on the topic but that's a nice broad overview of the Wikipedia. I'll let you dive in more from there if you're curious. It can lead to some interesting destinations.