A chalk reef, believed by the Joint Nature Conservation Committee to be the longest in Britain, has been discovered off the coast of Norfolk.
The reef, found by divers Rob Spray and Dawn Watson, stretches for 20 miles, from Cley Next The Sea to Trimingham.
When the pair set out to map the underwater biodiversity the true extent of the reef was revealed.
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 was 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."
Mr Spray had been collecting information about the underwater life as part of the Seasearch initiative organised by the Marine Conservation Society (MCS) when he made the discovery.
The pair are now planning to map out how wide the reef is.
Beth Stoker, of the Joint Nature Conservation Committee, said she did not know whether it was the longest chalk reef in the world but it was "certainly the longest in Britain".
She said: "The chalk reef off the coast of Thanet (in Kent) was previously the longest recorded chalk reef in Britain, but based on the MCS survey work it would seem that the chalk reef off the Norfolk coast is substantially longer."