Infrasound is too low-frequency for humans to hear. It has been mistaken for ghosts, has been known to cause nausea and headaches in humans, and is used to monitor the testing of nuclear weapons.
It's difficult to write about infrasound, because while some people consider it just an ordinary wavelength of sound, others have considered it magic. For every semi-credible link, there's a link to a bigfoot page, or a magic healing center.
Infrasound is like any other sound wave. Something vibrates, and the vibrations pressurized the air around the object. That pressure travels through the air until it hits an eardrum. The number of pressurized waves that hit an ear per second determines the pitch. A high frequency will produce a high note. A low one will produce a low note.
The lowest frequency that humans can hear is about 20 hertz, or twenty waves per second. Anything below that is infrasound. It doesn't register as sound, but it still does jam its way into a human ear, like that worm in Wrath of Khan, and it can be felt.
How? Like the worm in Wrath of Khan - not pleasantly. Infrasound is said to cause headaches and nausea that can last for days. At other times, it supposedly results in a feeling of generalized dread, sorrow, or fear. Some people have even reported seeing or feeling the presence supernatural entities after seeing when influenced by infrasound.
That infrasound is not without tangible proof. Elephants use it to communicate with each other, as do whales and giraffes. It's even said that whales use blasts of infrasound to stun prey. The waves are real, and there's no doubt that they can be heard and used by living things.
But the effect is still sometimes disputed. So many things create infrasound. When winds pass over mountain ranges, or waves crash down in bays, or earthquakes begin, infrasound can be created. Some people even say that they can feel it when they look at the northern lights. Maybe all the depression and anxiety in the world can be chalked up to nature screwing with us.
Fortunately, the human race has developed enough that we have a chance to screw back, and to do it with a certain poetic vindictive irony. Nuclear bombs are one of the most reliable man-made producers of infrasound. And infrasound is one of the most reliable ways of figuring out whether or not a country is testing nuclear bombs.
So if you feel anxious, nauseous, or sorrowful, be sure to pick up the phone and call your local branch of the military. I'm sure they'll be thrilled to know you're doing your part for non-proliferation.