A strange black ring has been spotted hovering over rooftops in Nottingham following an explosion, leading to wild speculation as to what it might be - ranging from an inter-dimensional portal to a UFO.

The video emerged after an explosion at a nearby industrial estate filled the sky with black smoke, which appears to have formed the black ring.

While some Youtube commenters offered outlandish theories, others gave a more sober analysis, saying it looked more like a smoke ring than an alien spacecraft.

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A giant smoke ring (pictured) was seen over Cotgrave, Notts, on Thursday evening after a fire broke out at Hollygate Industrial Park just before 8pm. Amazed locals captured the phenonomenon after hearing a loud explosion

In the video, the strange ring drifts slowly over rooftops in a residential area.

One voice can be heard saying 'I've never seen anything like it,' while another can be heard asking 'are we being attacked by aliens?'

Nigel Watson, author of the UFO Investigations Manual, told MailOnline: 'It's an excellent video of what is probably a smoke ring.

'In the past these have been caused by fireworks testing or accidental explosions. They are created when there is a blast inside a circular container or structure like a steel drum or factory chimney.'

After the video was uploaded, reports of an explosion began to emerge at Hollygate Industrial Park in Cotgrave, Nottinghamshire.

More than 30 firefighters were battling the blaze last night and Nottinghamshire Fire and Rescue Service advised those living nearby to keep doors and windows closed to keep out the smoke.

STRANGE BLACK SMOKE RING SEEN OVER KAZAKHSTAN The smoke ring spotted in Nottinghamshire follows reports in April of a similar black ring seen in Kazakhstan. Within minutes of the video being shared online, YouTube users began to question whether the thick black hoop was an alien spacecraft, while others suggested it might just be cigarette smoke immediately in front of the camera but giving the perception it is in the sky. Speaking to Russia Today, Andrey Solodovnik - associate physics professor at the Northern Kazakhstan State University - dismissed suggestions the cloud was caused by aliens. He said that based on similar clouds seen in the skies in recent years, the giant black hoop was likely to have been little more than a smoke ring generated by combustion at nearby factories. Advertisement

While on Youtube, the debate raged on as to what caused the black ring.

One user claimed it was an 'interdimensional portal', while others offered more scientific theories as to what might have caused it.

'Too bad you didn't have a drone. I'm pretty sure you would get points flying through it,' said Youtube user GranVlog.

'That is a toroidal vortice, quickly rotating cloud of gas and smoke,' said user MYeahman. 'Exactly the same as the rings you see dolphins, smokers and even volcanoes produce.'

The smoke ring spotted in Nottinghamshire follows reports in April of a similar black ring seen in Kazakhstan.

Speaking to a local news website villager Oleg Menshikov said: 'It was like a black cloud. We saw it at around 4 pm on April 3. It dissipated like smoke, but it was completely odourless.'

Within minutes of the video being shared online, YouTube users began to question whether the thick black hoop was an alien spacecraft, while others suggested it might just be cigarette smoke immediately in front of the camera but giving the perception it is in the sky.

Others thought the cloud was likely to be a naturally occurring weather event known as a thermal microburst, which is caused by a falling mass of warm air.

A smoke circle (pictured) caused by a large explosion has been spotted over Nottinghamshire. Last night more than 30 firefighters were battling a large fire a Hollygate Industrial Park, in Cotgrave

Experts believe the cloud was roughly 100 m (328 feet) in diameter and floated close to a mile in the air.

Speaking to Russia Today, Andrey Solodovnik - associate physics professor at the Northern Kazakhstan State University - dismissed suggestions the cloud was caused by aliens.

He said that based on similar clouds seen in the skies in recent years, the giant black hoop was likely to have been little more than a smoke ring generated by combustion at nearby factories.

Plenty of industrial machinery is known to create ring-shaped clouds of smoke, though these are usually lighter-coloured, considerably less dense and vanish in just one or two minutes.

Just driven past this giant smoke ring in #Nottingham. Anyone know what caused it? Some kind of explosion? pic.twitter.com/BL5G9kMDBm — Kelly Cookson (@kellycookson) July 23, 2015