IT is the X-Files-style mystery that has been puzzling people for months.

What were the strange black triangles spotted flying silently over Southampton landmarks?

Caught on video three times in the past few weeks, it caused a buzz among UFO enthusiasts from across the country who have described it as the best they had seen for some time.

It even sparked an investigation by Southampton Airport after a report from a worried member of the public.

Thousands of readers logged onto the Daily Echo website to watch the slow moving objects twist and spin over Millbrook, Shirley and the docks.

Now we can reveal what we think was behind the phenomenon: A solar balloon.

The mini-hot air balloon can be made with just four black bin bags, sticky tape, scissors and a hairdryer.

Mix those ingredients with a warm, sunny day and you have a black triangular balloon which could easily be mistaken for one of the UFOs which have been spotted over the city.

Click here to watch a Solar Balloon in action

Unfortunately, the Daily Echo's launch attempt yesterday was not too successful - an unseasonably cool and cloudy summer afternoon meant our solar balloon lifted only a few metres before crashing back to Earth.

However, Michael Lowe - organiser of this weekend's Southampton Kite Festival - agreed with the findings of our investigation.

"My opinion is that this is neither a kite or a UFO,"he said.

"I think that someone, somewhere has made up a large black plastic square-shaped balloon that is inflated by the action of the sun on the trapped air inside and let it go."

"The minimal weight of any tether line - which you would not be able to see by eye at altitude - would keep it stable, allowing it to float away on the slightest of breezes.

"These are different at different heights so even if you could feel no air movement at ground level, there could be movement at a higher level.

"You often see the long tubular types of this for sale, easy to make and cheap.

I really think that this was just a variation on that theme.

"There might even be something similar at the Kite Festival this coming weekend, you never know."

Freemantle resident Gary Moore contacted the Daily Echo to say he had a similar triangular balloon fly over his garden recently.

"It came right down over our roof, my wife spotted it out the window," he said.

"It was made out of a type of vinyl material and it was covered in dirt and mould. I would love it to be a UFO because I am a believer, but what I saw was not a UFO."