These intriguing round marks in the heath at Studland are definitely not crop circles – but what they are remains a mystery.

Nestling in the heath near Poole Harbour, the 80 circles with an average size of 20 metres across are obviously man-made and have 2ft high turf walls around them.

Fifteen possible explanations for the mysterious marks were put forward at a recent Poole seminar, where audience members came up with a further five.

Research carried out by the Poole Harbour Heritage Project reveals that the circles, which run parallel to Ferry Road on the east side, were first recognised in 1860 and thought to be prehistoric hut circles.

“One of the theories is they were a shower of meteorites,” said Bernard Dyer, chairman of the project. “Why they should go there in one group, I don’t know. They look rather more man-made than that.”

Some hold water and some have entrances and they are thought to be at least 300 years old.

Another interesting theory is they were dug out in the 17th or 18th century as weathering pots for seaweed. This is plausible as with rain washing out the salt, seaweed would make good fertiliser for the poor soil of the Purbeck hills.

As smuggling was rife along the Dorset coast in those days, it has also been suggested they would make good hiding places from the revenue men for barrels of rum and brandy.

“A couple of revenue men are not going to search all of them. That’s an intriguing possibility,” said Bernard.

Theories range from seasonal huts for fishermen, military training in the Napoleonic War, duck decoys, gravel pits, sheep dips, herb or bee gardens, salt pans, connected with copperas (iron sulphate) production, peat cutting and clay pits.

They are on the oldest part of the South Haven peninsular, on land belonging to the National Trust.

Martin Papworth, regional NT archaeologist said salt workings were mentioned in the Domesday Book.

“There is no real documentary stuff,” he said. “They look like they are some part of an industrial process.”

Tim Darvill, professor of archaeology at Bournemouth University, said: “The whole point about the Studland Circles is that we don’t know what they are.

“Some kind of industrial or agricultural usage is most probable and personally I favour them being drying platforms for fish or seaweed or peat.

“The challenge is to find out what they are, how old they are and who built them,” he said.

If anyone has any theories, PHHP would love to hear them. Contact Bernard Dyer at 6 Weston Road, Poole BH13 7BN.