A series of mysterious, apparently inexplicable explosions heard around the UK were eerily reminiscent of a futuristic 'pulse detonation jet' an expert has said.
The strange bangs were reported around the UK (and as far away as New York) on 29 November, with residents from Aberdeen to Croydon noting the blasts on Twitter.
Dr Bhupendra Khandelwal told the Mail that the sound -- which can be heard below -- sound like the type of engine on which he is working at the University of Sheffield.
"It makes the same kind of pulsing sound as the one on this audio," he told the site.
"When we run a test engine it’s a real industrial noise and you can hear it for miles. We have people coming to us asking to make less noise or keep it to the daytime."
However he added that the sound could have easily been something else - and that half an hour was a "bit too long" for such an engine to cause the noise.
The firework-like sounds occurred between 10pm and 10.30 pm on Saturday.
The Ministry of Defence said no supersonic jets had been scrambled at the time the blasts were reported. Meanwhile the Met said that aside from a firework display in Croydon, there was no obvious cause.
The reports became confused after another report of an explosion occurred in North Yorkshire hours later, but the two incidents seem unrelated.
So what was it?
- The MoD has ruled out British fighter jets (none were scrambled at the time)
- The Met Office says "it wasn't meteorological"
- The Met police said fireworks in Croydon took place - but there were no such displays in other locations
- Booms were also heard in New York at the same time
- The reentry of remaining bits of the Russian satellite Kosmos 2251 were scheduled (details on this event are scant) at the wrong time to reach the UK
- There were no visual sightings of meteors
- Conspiracy theorists have emerged with old tales of an Aurora jet - but that is pure rumour at best.
Totally coincidentally, Lockheed Martin today unveiled a new super-speed jet capable of reaching LA from New York in two hours.
Currently it's not possible to say what the blasts were. But did you hear it? If so let us know on Facebook and we'll add your story to our report.
Also, if you happened to be flying a secret US pulsejet over the UK at the time, that would be handy to know also.