© Cornwall LiveSt. Ives, England.
Dozens of people have taken to social media to query the source of the 'sonic boom'.
It happened at around 11am this morning, with reports of the noise being heard in Carbis Bay and Halsetown.
Steve Cross, who owns Cross Estates in the town, said: "It was a massive explosion. Our office shook. I don't know what it is. A sonic boom, possibly.
"It happened at about 11-11.30am. The windows shook. Proper shake. The windows shook and doors shook. It was a big old bang. A single big boom."
Reports are continuing to come in across St Ives. We'll bring you live updates as we have them.
© Cornwall Live
The other potential cause, according to those who heard it, is that it could have been a 'sonic boom'. That's the sound associated with the shock waves created by a object traveling through the air faster than the speed of sound.
They generate considerable amounts of sound energy and experts say that they often sound like an explosion to the human ear.
Was the noise caused by a 'bird scarer'?
Some residents have reported that it could have been caused by someone trying to clear birds or animals in the sea away from an area, presumably to fish. But this has been dismissed by St Ives Harbour's maritime assistant Colin Walker who says that it's unlikely a bird scarer could have made such a noise.
"A complete mystery"
Colin Walker, Maritime Assistant at St Ives Harbour, says he has no idea what caused the boom.
"It was quite a loud bang", he said. "We've heard that it shook windows and was heard far away, but there's no reports of any smoke or anything like that.
"It's a complete mystery. The National Coastwatch Institution don't know what it was either, they're looking into it and will notify us when they know more and vice versa."
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