To many, they are nothing but pranks that damage crops.

To others, they are the mysterious harbingers of tumultuous events.

Whatever your view, there is no denying the beauty of crop circles. The intricate, swirled patterns which have appeared in fields across the world have been studied for years.

Despite some people owning up to creating the circles as "a lark", crop circle author, Andy Thomas, claims so far no one has come up with sufficient evidence to completely explain them.

Mr Thomas and colleague Geoff Stray, will give a lecture on the phenomenon in Benfleet in March.

Mr Thomas, 41, from Lewes, in Sussex, first became interested in crop circles 16 years ago after seeing one in the West Country.

He has since become convinced some cannot be dismissed as pranks.

He said: "The thing is to look at the evidence. When you do that, I believe for every formation that is man-made, there is also a significant proportion that cannot be explained."

About 250 crop designs appear around the world each year. Even the Salvation Army farm, in Hadleigh, has not escaped them.

The origins of the often complex and beautiful patterns found swirled into crop fields around the world remains a complete mystery.

Several thousand have been documented since records began centuries ago.

Mr Thomas said: "If you know what you are looking for, you can see which are man-made and which are unexplained. You can usually see where a crop has been manually crushed, but you don't see any evidence of that in some crop circles."

He claims: "Some people have watched circles appear out of nowhere in a matter of seconds.

"There is also evidence of biological changes in the plants. For instance, the seeds, particularly when they are young and green, are notably mutated as if some sort of heat has been applied."

"Even farmers admit that not all are man-made."

Crop circles are a global phenomenon, but predominantly appear in the northern hemisphere, with southern England - particularly Wiltshire - a particular centre of activity.

By and large they appear from spring until early autumn, with most arriving during the three main summer months.

The growing sophistication and size of the circles leave many convinced there is an unexplained force at work.

Mr Thomas said: "Nobody is quite sure why they appear.

"Many people think there is some kind of intelligence at work.

"I believe something wants us to look at the way we live and look at the world around us."

The lecture by Andy Thomas and Geoff Stray will be at Richmond Hall, Richmond Avenue, Benfleet, on Sunday, March 25, from 10.30am to 3pm.

Tickets cost £8 pre-booked, or £10 on the door. For more information, call 07773 560378.