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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.

Other people have proposed the sounds might be being caused by cloaked UFOs or Fracking, a controversial method for extracting natural gas from the ground.

Nick Pope, who used to investigate UFOs for the MoD, believes that although there is unlikely to be one single cause, something more mundane will certainly explain some of the sounds.

He said: "One possibility is that we're dealing with some sort of phenomenon involving thunder. Sound can behave in very funny ways.

"Another option might be power lines. Again, under some circumstances we know power lines can make a humming and crackling sound. That may well explain some of these events."

Some have also suggested the apparent atmospheric rumblings might be the product of nearby freight trains or high-speed rail links, though it is not known whether any of the videos were filmed near railway lines.