Gerald O'Conner, 47, who lives on Tofts Grove, Rastrick, said he was doing a spot of gardening during lockdown when he found the six-foot wide 30-foot deep well.
Mr O'Conner is now toing and froing with Calderdale Council to get the well filled-in and restructured after being told he and his neighbours must not use the access point to his back garden due to health and safety concerns.
He said: "I was digging up the land, which is shared with my neighbour. When I came back to it the next day there was a huge hole, which I could've fell into at any point.
"I've lived in my home for 18 years and I was never told there was a well underneath it. I did a bit of digging and found out there was a mill on the plot of land before my house was built in 1989, as well as a reservoir less than 200 yards south-west of the well, which I suspect runs under my home too."
Mr O'Conner spoke of his initial reaction.
He said: "I said 'holey-moley', I was in shocked. When you come across issues with your home you usually call the plumber or electrician; who I call for this? It's been stressful because this will limit me now if I ever want to sell my home in the future, and this will definitely stop any sale going through and drop the value.
"I no longer have access to the rear of my home, and I have a two-year-old granddaughter, which I have to make sure stays well away from it. My neighbours and I have cats which we'll also have to constantly monitor. Oddly enough I've considered starting a mini landfill business and making use of the well.
"The council came to look at it in June and haven't been back since. It's not safe and I want it taken seriously."
Calderdale Council’s corporate lead for planning, Richard Seaman, said: “The issue was reported to the council on June 8 and officers visited the same day to assess the situation and cordon the area off with barriers and netting.
“Officers established that the hole did not pose a risk to either the building or retaining wall and as it is on private land, advice was given to the owners that they should contact their insurance companies.”
A spokeswoman from Yorkshire Water said: “The sink hole in Mr O’Conner’s garden seems to be a well, which was probably part of the original mill building that stood on the land.
"Our team went to investigate the site to confirm that it's not connected to any of our clean water or sewer pipes and should not have any impact on local supply.”
Member of the Rastrick Local History Group, Andy Eccles said: "The former Crowtrees Mill, which sat on the site, was owned by the Clay family, who were textile manufacturers from the mid-1800s in Rastrick.
"The founder of the mill, Joseph Travis Clay’s son, Arthur was chairman of the Rastrick School Board in the late 1800s and the family lived at Holly Bank House on Field Lane, which is about 200 metres from where the well has been found.
"The stream that filled the well comes up in the fields above Ainley Top and used to fill the mill dam of John Smith’s mill on Dewsbury Road before flowing under the cricket field at Badger Hill and then filling Spout Mills dam further down New Hey Road.
"The stream goes under Delf Hill and then between Crowtrees Lane and Tofts Grove where the water was used to fill this third mill dam at Crowtrees Mill.
"It meets up with another stream near to Bramston Street where it continues down to the River Calder at Bridge End, Rastrick."
The mill was eventually taken over by Sladdins, manufacturers of shoulder pads.
The business had originally started in 1894 and they moved to Crowtrees Mill after Clay’s had closed down.
For the next three decades they supplied vast numbers of shoulder pads to the tailoring and clothing trades. Always known locally as just ‘Sladdins’, the changes in fashion and failure to develop alternative lines forced the company’s closure in the late 1960’s after which the mill stood empty for some years before being demolished.
For more info, visit: www.myrastrick.com