Sound with frequencies below about 20 Hz is known as infrasound. Such sounds are not audible to humans.
Infrasound is produced by various natural events, including earthquakes, storms, and ocean waves crashing on shores. Man-made sources of infrasound include chemical and nuclear detonations, aircraft sonic booms, and wind turbine farms.
Some animals make use of infrasound in their communication. Elephants use frequencies down to 14 Hz (or possibly lower). With atmospheric absorption of sound being negligile at these frequencies, infrasonic communication over 5 km is possible and has been observed with elephants. Sound can propagate even further underwater. Whales, using frequencies between 10 Hz and 30 Hz, can communicate over hundred of kilometers.