The cause of a loud bang heard in Leicester and on the outskirts of the city has perplexed residents and authorities alike.
A "large, rumbling, explosion, thunder-like sound" was reported by people living in the city centre, New Parks and Beaumont Leys on Sunday evening.
Similar reports also came in from residents in Birstall, Rothley, Glenfield and Enderby.
Twitter has been awash with residents saying they heard the mysterious bang, with rumours abounding about its possible cause.
One person, with the username @EllingtonWright, tweeted: "Yep, heard that last night. Two loud rumbling booms about 30 seconds apart.
"Very strange indeed."
Another tweeter, who goes by the name @plot44, wrote: "Heard it in Birstall. Sounded like multiple explosions."
Twitter user Lindsay Clarke wrote: "Also heard it in Birstall... could not figure out what it was."
@MissButler2_ wrote: "Everyone heard it, people in Birstall, Enderby, Glenfield. The police said military aircraft... We say aliens!"
A majority of the reports said the mysterious sound happened at about 8.30pm.
Twitter user Alex Barston, from Leicester, wrote: "I was listening to music at that time. However, several friends who live in the area quickly posted things on Twitter about what they heard.
"These ranged from 'loud thunder' to 'gunfire' and said it caused things to fall off shelves."
Despite the interest in the mystery sound, a police spokesman said only one person reported the bang.
He said: "We received a single report from a member of the public in Glenfield at about 10.30pm on Sunday, who said they had heard a 'large noise'.
"We looked into it and contacted the fire service, East Midlands Airport, councils and other agencies, but found no information that would explain the sound reported to us."
One theory is the thunderclap could have been a sonic boom from an RAF jet on a nighttime training exercise.
No one at the Ministry of Defence was available to comment at the time of going to press.
However, a spokesman for air traffic control at East Midlands Airport said: "We would know about any Ministry of Defence flights in our airspace.
"We have gone through our flight plans for Sunday evening to see if there was any unusual activity.
"However, there was nothing at all out of the ordinary that would make that kind of noise."
A fire service spokesman said: "The only calls we received between 8.30pm and 10.30pm on Sunday were to a house fire and a bin fire but nothing whatsoever that would explain the noise being reported."
A city council spokeswoman said: "We received no reports of sonic booms or loud, unexplained explosions on Sunday night."
A Leicestershire County Council spokesman said a member of staff living in Glenfield reported hearing the sonic boom but the council could not explain any activity that could have caused it, including blasting at granite or opencast coal mines in the west of the county.
Another possible explanation could be the sonic boom from a meteorite falling to Earth.
Scientists with the British Geological Survey are looking into the reports.
A spokesman said: "If there was indeed a sonic boom, whether from an aircraft or otherwise, in Leicestershire, our equipment will have picked it up."
