Warning over Dartmoor's hidden mine shafts
Mine shafts up to 100ft (30m) deep have been weakened by severe weather and could collapse, Dartmoor National Park Authority has warned.
Winter storms are thought to have damaged the wood and rubble used to cover the shafts, some of which are unfenced and unmarked.
The workings, which are often covered by grass, are "difficult to detect", the authority said.
Previously, sheep and ponies have had to be rescued from shafts on the moor.
Head ranger Rob Steemson said: "We are aware of a few collapses this year and the sites are being regularly checked and monitored by the landowners."
Once found, the areas are fenced off and warning signs erected.
Mr Steemson said: "Unless you really know what to look for they are difficult to detect to the untrained eye.
"Landowners and ourselves are concerned that after the bad weather a few of these are falling in."
He said there were hundreds of covered tin mine shafts across the moor.
Mr Steemson added: "There's no need to panic, but walkers should use maps to be aware of the mined areas."
Barry Gamble, mine historian, said: "Over time they [the shafts] became deeper as the pumping technology improved and some have been insubstantially tapped.
"The rain causes substantial weight on the wood and rubble and dislodges them.
"Movements can then lead to a sudden collapse."
The Duchy of Cornwall, which is a significant land owner on Dartmoor, said key mine shaft areas were fenced off and periodically inspected.
A spokesman said: "However, there is of course a risk of further collapses, but as and when they are reported we act immediately.
"We have an expert who advices us on the likelihood of collapses."
Since 2004, Mr Steemson said he was aware of up to 15 that had fallen in and had been fenced off.
In February, the Met Office announced the UK had had its wettest winter since national records began in 1910.