The online community have been spooked by a series of videos uploaded to YouTube and other video sharing websites which appear to show loud noises rumbling apparently from beneath the ground.
People from a wide ranges of cities and countries have uploaded videos.
While some appear to be relatively obvious hoaxes, others remain unexplained and ideas as to what the noises might be range from the mundane to the arguably ridiculous.
Some people on social networking sites, online forums and video sharing sites have suggested the strange noises might be the work of HAARP or High Frequency Active Auroral Research Program. HAARP is a US-based research project that studies the ionosphere - a part of the Earth's upper atmosphere - and it has regularly been the subject of conspiracy theorists' discussions.
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