In the most recent case, two pensioners, John and Eileen Cooper, were woken by what they thought was thunder to find a 15ft- wide (4.5m) hole outside the front of their bungalow at Ashton, near Helston.
The hole is the entrance to Harvey's shaft, part of a tin mine which closed in 1880. The shaft could be several hundred feet deep. The Coopers fear the hole may soon threaten their home. 'If the top of the hole goes any further it'll hit the septic tank,' Mr Cooper said.
Last week another shaft collapsed at Gunnislake, only yards from the site of an incident in June when the 756ft-deep (230m) Michael's Shaft collapsed.
Cornwall County Council has also recently discovered old mineworkings underneath two primary schools, which will remain closed next week while remedial work is carried out.
Tony Elliott, a mining surveyor, believes that prolonged dry weather, followed by August's downpour, has contributed to the mines' instability. 'Many of these old mines closed at the same time towards the end of the last century,' he said.
'In most cases they were simply capped with the timbers from the mine. Those timbers have now rotted, so it appears the shafts are all collapsing together. If that is the case it could be very serious.'
He estimates there are more than 15,000 shafts in Cornwall, some nearly 200 years old, many of them not shown on any map.Reuse content