Mysterious sounds have been heard booming from the sky all around the world - in some cases they were so loud they set off car alarms. The unsettling noises were heard recently from Europe to Canada, sounding like groans and powerful horns. In Germany noises coming from the sky were recorded on a video camera and uploaded to YouTube, with car alarms clearly heard going off in the background.

But experts have said that there are rational explanations. University of Saskatchewan physics professor Jean-Pierre St. Maurice told CTV that Geoscientist David Deming from the University of Oklahoma, meanwhile, has previously written about a phenomenon called The Hum - 'a mysterious and untraceable sound that is heard in certain locations around the world by two to ten per cent of the population'. Writing in the Journal of Scientific Exploration, he said that sources of The Hum could include telephone transmissions and 'aircraft operated by the U.S Navy for the purpose of submarine communications.' According to NASA, the Earth has 'natural radio emissions'.

The Agency said: 'If humans had radio antennas instead of ears, we would hear a remarkable symphony of strange noises coming from our own planet. Scientists call them "tweeks," "whistlers" and "sferics" They sound like background music from a flamboyant science fiction film, but this is not science fiction. Earth's natural radio emissions are real and, although we're mostly unaware of them, they are around us all the time. For instance lightning can produce eerie-sounding radio emissions, NASA added. Earthquakes can also produce sub-audible sounds, according to seismologist Brian W Stump from the Southern Methodist University in Dallas. -Daily Mail