Scuba divers have discovered what is believed to be the world’s longest chalk reef off the Norfolk coast.

The band of chalk features “moonscapes” and a “mini Stonehenge”, and is home to crabs, lobsters and a variety of fish.

Local fishermen always knew there was chalk in the area, but it was only when diver Rob Spray, 43, and his partner Dawn Watson, 40, set out to map the underwater biodiversity that the true extent of the reef was revealed.

At first the pair thought the feature was a mere five miles in length, but over the course of about 50 dives, they discovered the chalk stretched for 20 miles, from Cley Next The Sea to Trimingham.

In some places it is possible to walk off the beach and snorkel the reef just 50 metres from the shoreline.

Mr Spray, from Sibton in Suffolk, who has been diving in the area for 12 years, said: “We thought there was a five-mile stretch but throughout the season as we kept diving we kept finding more. It was tremendously surprising. On land everything has been discovered to death. But this is a habitat you can snorkel.

“There are areas of polished moonscape, boulder fields, and the most spectacular thing we discovered were two-metre high arches which look like a mini Stonehenge.

“It’s a much more exciting habitat to look at than the North Sea, with its acres of sand. There are crabs in holes and lobsters running around on the surface.

“On a good day it’s almost like a Mediterranean dive. And it’s literally just under the surface. You can walk in off the beach and start diving in less than a metre of water. As a crab-hugging conservationist I would like to see it protected appropriately.”

Mr Spray and Ms Watson said they believed it to be the longest chalk reef in the world but added that measuring such features is an expensive and complicated science.