A mysterious "boom" that has been ringing out in the middle of the night in a US neighbourhood has police baffled.
The loud noise, described as an explosion or gunfire, has been waking up residents in the city of Denver, Colorado, for months.
Wheat Ridge police have received dozens of calls about the bizarre sound, which typically happens once around 3am, mostly on the weekends.
Residents told local media CBS4 the noise was loud enough to disrupt their sleep.
"You can hear it a ways away, but it's loud enough to wake me up," Sara Anthony said.
Denver is the latest city around the world to hear a mysterious boom.
Thumping noises have been recorded everywhere from the Middle East, to the US, to Cairns in Queensland over the past two years.
In Wales, a series of booms heard in May 2017 went unexplained.
"It nearly gave me a heart attack it was that loud. At first, I thought it was shotgun blast or a firework, but it was way too loud for that. It sounded more like a tank going off," one Abergavenny resident told local media.
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In October 2017, a similar sound had Cairns locals stumped.
A FA-18 Hornet plane was heard flying over Cairns the previous night, but that is not thought to be the cause.
Resident John Romanov found a smoking hole in the ground in bushland, leading to a theory that the bang was a small meteorite that fell from space.
Then, just two weeks later, another boom was heard over the Eyre Peninsula in South Australia at the same time a blue meteor passed across the sky.
"It just got bigger and bigger and it was just this big flash across the sky and there were sparks coming off it," Port Lincoln local Lisa Watson told News Corp.
Residents were woken up in the middle of the night in April, 2016, after hearing several loud bangs.
Locals of Massey, Ranui, Te Atatu and Henderson reported hearing a loud noise at around midnight that "shook the walls".
People took to Facebook to describe the noise, saying it sounded like an exploding tyre or fireworks "but louder".
No official explanation was given.
Just last month, another mystery boom rocked the US neighbourhood of North Oak Cliff, Texas.
Describing it as the loudest thing he has ever heard, Phillip Washington said he could feel the noise in his apartment
"Out of nowhere, just this pow…just this huge explosion," he told CBS News.
"It was without a doubt, the loudest sound that I've heard."
While some suspected gunfire, fireworks or even a car backfiring, those closer to the mysterious boom said it was something far more intense.
Ron DiIulio, an astronomer at the University of North Texas, said the noise was likely from a sonic boom caused by either a meteorite or some sort of space debris.
"It was immediately apparent that something had fallen from the sky," he said.
The scientist said whatever entered the Earth's atmosphere was likely only about the size of a grapefruit.