A curious archaeologist discovered a huge mineshaft under his patio after it began to sink.
Simon Timberlake, 49, began excavations out of professional interest when he noticed the back garden subsiding.
The mineshaft drops at least five metres into a chalk bed and opens up into a cavernous space below the patio in Fen Ditton, Cambridgeshire.
Mr Timberlake said now the mineshaft had been secured his timber-framed home was in no danger of being swallowed up.
Mr Timberlake said he had been amazed by what he found.
"It's not just unusual, it's probably the only shaft of this type in the area - Cambridgeshire County Council's archaeologists are really scratching their heads.
"The reason it is unusual is because it's a mine rather than a quarry."
The chalk would have been mined and used in construction.
Mr Timberlake, who normally works at prehistoric metal mine sites, said: "The shaft is cut straight through the chalk, but because the mine wasn't filled in properly, we've noticed it's there and uncovered it again.
"I live in an old timber-framed cottage, and I believe this mineshaft could date from a similar time - which means it could be the work of miners back in the 17th Century."
Mr Timberlake said the mine had been filled in with rubble and sand and a false base made of posts and netting held the debris in place.
Preserved shark's tooth
But as the decades passed, the netting deteriorated and the contents spilled out into the bell-shaped cavern, causing the patio above to subside.
Mr Timberlake said he was never in danger of falling through, but he will be spending about £1,000 to fill it with concrete - though he is planning to leave the last few feet and the rim of the shaft open.
He said: "I've found the whole thing fascinating, and it is part of the local history, so I'll keep it open to show to people.
"It's thrown up some really interesting oddities, including lots of fossils - the best one I've found is a perfectly preserved shark's tooth - something you don't find in your garden every day."