Any sound waves which has frequency lower than 20hz is called Infra sound. Its study is called Infrasonic. Humans can only hear the sound which are in the frequency 20hz to 20000hz. Infrasonic waves can carry over long distances and are less susceptible to disturbance or interference than higher frequencies. Many animals use infra sound to communicate with others. For example, the elephant uses the infra sound to communicate with other herds which could be even 100 miles apart. The legs of the elephants senses this sound.
The first official observation of infra sound was in 1883 when Krakatoa erupted most violently. it is believed that the sound was heard unto Perth in Australia and in Mauritius which is 3000 miles away from source. This volcanic eruption also emitted infrasonic waves which shattered windows hundreds of miles away from the volcano.
Many natural phenomena like earthquake, fire, volcano, flood emit infra sounds. Some earthquakes emit infra sound hours before the actual earthquake. That's why we can see the animals in zoo became impetuous in the recent earthquake. Actually infra sound are used to in seismography to predict and measure the earthquake. The interesting fact is that the earth surface acts as a medium through which infra sound travels.
Even though humans cant hear these sounds, we react to these sounds. These sounds produce a sensation of fear, awe, extreme emotional distress. It is found that in haunted houses and places, there is high infra sound production which creates that sense of fear. Vic Tandy did research in these haunted places and found these facts. The ghosts, the people see are the optical illusion produced by the resonance of eyes. The resonating frequency of eye is 18 Hz. Humans tend to become numb when exposed to high doses of infra sound.
Infra sound is used by whales to stun their prey. At some tonal angles, the prey gets blasted. During world war, It was used to identify the presence of the artillery.
A few links where you can get more details on infra sound