The formation of the Giant's Causeway may have taken just a few days rather than thousands of years, according to a new theory.

Dr Mike Simms, curator of natural sciences at National Museums Northern Ireland, has put forward the first new interpretation since 1940.

The Giant's Causeway is Northern Ireland's only UNESCO World Heritage Site and is operated by the National Trust, which has acknowledged Dr Simms's theory.

The origin of stone formation, which have attracted visitors from across the world, include theories ranging from them being built by the mythical giant Fionn Mac Cumhaill to more scientific explanations.

To mark UNESCO's International Geodiversity Day today, Dr Simms considered why the extraordinary geological features are found at sea level only.

The scientific explanation, widely accepted for decades, is that a river valley was filled with lava that ponded to a greater depth than normal, and as the thick layer of lava cooled and solidified, it formed these regular columns.

However, Dr Simms has put forward the idea that if this lava-filled hollow was a valley, it would have cut through old layers of lava beneath.

He has identified layers of old lava on the shore either side of the Giant's Causeway sloping inward where older layers have not eroded away.

"An analogy I find helpful when explaining this involves cake. Eroding a river valley is rather like cutting through a layer of cake to reveal layers beneath the surface," said Dr Simms.

"In my interpretation, what we actually see are layers of older rock sloping towards the Causeway - more like a badly baked cake that has sunk in the middle.

"I believe the ground subsided as lava moved up and erupted at the surface.

"The lava filled the depression creating the conditions for the columns to form. This event likely took just a few days rather than the many thousands of years that would be needed for erosion to create a river valley," he said.

Dr Simms first started thinking about the theory while leading a field trip to the World Heritage Site in 2012.

"I owe a particular debt of gratitude to another geologist, a young Brazilian man who was on (the) field trip.

"He questioned how long it would take to erode the supposed river valley, and it was this that opened my eyes to the evidence before me.

"I had visited the Giant's Causeway many times before, and until that moment I had just accepted the previous theory.

"It goes to show that even at world famous landmarks such as this, new discoveries can be made from a simple observation."

Max Bryant, general manager at Giant's Causeway and Carrick-a-Rede, said there can always be new possibilities and perspectives waiting to be discovered.

"It highlights just how marvellous, magnificent and mysterious a geological formation we have here in Northern Ireland to share with the world," he said.