Down a set of mossy steps in Helen Jackson's back garden lies a cellar that helped lead the way to Sheffield's old industrial might.
It seems a bit damp, dark and unprepossessing at first - but once illuminated with a lamp, the site's value becomes clear.
The cellar, at Helen's home on Top Side, Grenoside, is a rare example of an early five-hole crucible steel furnace, dating from 1797 and relatively undisturbed since the plot was sold in 1869.
Part of a family-run village business known as Grenoside Steel Works, the furnace produced the metal that made items ranging from nails and files to saws and wire, in the days before the big factories of the city's East End became the trade's dominant players.
Next weekend visitors will be able to tour the cellar as part of the annual Heritage Open Days festival in Sheffield, which runs from September 7 to 10 - and, in Helen's view, it's a 'very important' place to look at.
"You don't get the feel of how it all started, until you see it on this scale," says the former Labour MP, who represented Sheffield Hillsborough for 13 years until 2005.
"There are many people in Sheffield whose fathers and grandfathers worked at Firth Browns, and British Steel, and remember the noise, the banging and the dangers of all that heat and pouring metal in the big works. But they are quite blown away by this, because it was the early days."
Helen will be joined on the tours by metallurgist David Dulieu, who lives nearby. He is an expert on the history of crucible steel, which offered the first method of making a consistently high-quality steel and was crucial in transforming Sheffield into a major metalworking centre of innovation.
The process involved placing clay crucible pots - each able to hold 15kg of iron or blister steel - into a charcoal or coke-fired furnace capable of reaching 1,600 degrees Celsius. The pots were removed after about three hours in the furnace, impurities skimmed off and the refined molten steel poured into ingots, or casts. The set-up at Helen's home would have consisted of a melting shop at ground level, with the cellar allowing access to the actual furnace for loading fuel.
According to English Heritage, which awarded the site a Grade II listing in 2012, the date 1797 is significant - the crucible steel process was only invented in the early 1740s by Benjamin Huntsman in Doncaster, meaning the cellar is older than any other known surviving example, of which there are only around 18.
"It is quite extraordinary," says Helen, who has lived at Top Side for 12 years.
"As soon as English Heritage came down, there was a 'wow' sound."
Features of the furnace are still clearly visible. In the 13-metre long, two-metre high cellar sit ash-pit recesses for five furnace chambers, each located beneath the individual melting holes. The fire bars, where the crucible pots rested, remain too, and the stone floor has a shallow central channel which feeds water into a circular well near the entrance to prevent flooding.
Helen says working life in the cramped cellar was 'not pleasant'. A cellar boy would have been tasked with blocking and unblocking a hole in the chimney to regulate the temperature.
But David adds: "It was a four-hour production cycle, so they weren't rushing around all the time. The cellar boy would have had time to read The Magnet or The Boy's Own Paper - if he wasn't caught. Compared to some of the other operations it wasn't as demanding. On the other hand, a 12-hour shift is still long, six days a week."
The land at 2 Top Side was acquired by Richard Bayley and Jonathan Tingle in 1797 - by the early 1800s, Tingle was recorded as making a record output of two-and-a-half tons of steel per week at the furnace.
Jonathan died in around 1836. His business was taken over by his son, Benjamin, who died in 1865. Four years later the firm was bankrupt - beaten by thriving, large-scale steel operations in the Don Valley and elsewhere.
Helen says her approach to the cellar's conservation, based on sound advice, is to 'do nothing' and leave the environment untouched.
"I got it listed without any hesitation."
Some components have been lost - the former melting shop has been rebuilt as a double garage - but the furnace narrowly escaped disappearing completely. In the 1970s a planning application was lodged to convert the garage into housing.
"They would have had to destroy the cellar," says Helen.
Artefacts found at the site will be on display next week, such as ingot moulds and a charred-looking, used crucible pot complete with lid.
David says the Grenoside Steel Works 'represent a tipping point'.
"All the metallurgical activity in this area started as a money-spinning activity for seasonal workers. People up here who were farmers, quarry people, schoolmasters and iron founders thought: 'We could have a go at this'. Because it requires nous, and not too much capital. Once you've got the secrets, it's not too difficult."
The cellar will be open next Friday, Saturday and Sunday from 10am to 4pm. Call Helen on 0114 2463162 or email Jacksonh5@btinternet.com to book a place. See www.heritageopendays.org.uk for details of more events.