A wartime scientist has revisited the secret underground factory where she helped build aircraft engines 70 years since the end of the Second World War. Mary Cartwright was 20 when she was called up to work in the laboratory at Drakelow Shadow Factory in Kidderminster, Worcestershire, in 1943. She spent two years testing metal for Bristol aircraft engines in the site's underground laboratory, working as the only woman in her department. Now,  70 years after leaving her position, the 91-year-old great-grandmother has revisited the now defunct factory. Mary Cartwright revisited Drakelow Underground Factory in Kidderminster, Worcestershire, where more than 700 people worked during the war (right)  to produce aircraft engines. Now 91, the great grandmother tested metal for aircraft engines . Revisiting the site with her son, David, Mrs Cartwright said it was 'strange' to see it so quiet. She worked as the only woman in its laboratory between 1943 and 1945 . Built in 1943, the factory became known as Drakelow Tunnels for its vast network of underground paths spread over three-and-a-half miles. It was one of 26 'shadow' sites set-up to mass produce engines and parts for military aircraft and tanks that were built underground or into hillside so that production could continue if bombed. Designed by Rover at the behest of the Air Ministry, Drakelow employed 700 people when Mrs Cartwright began working there. Arriving every day at 8.30am, she used scientific formulas to analyse the ratio of metals in shavings given to her by 'the men'. Spanning more than three miles, the site became known as the Drakelow Tunnels for its vast network of secret passageways . The entrance to one of the tunnels which are built into hillside beneath Kingsford Country Park, Kidderminster . Another of the tunnels' entrances in Kingsford Country Park. While built at road level it is 300ft from the top of the hill . Set up by Rover under the instruction of the Air Ministry, Drakelow was one of 26 'underground' factories established to allow production lines to continue if the country came under fire . With more than 700 people working at the site at a time it was one of the largest underground factories during the war . Mrs Cartwright was sent to work there in 1943 after being called up at the age of 20. She had wanted to join the Civil Service beforehand . A stretch of offices in the underground factory. Their windows were high up so that staff would not be distracted by co-workers walking by . The kitchen where staff would have eaten during one of the long, underground shifts. Mrs Cartwright had no previous experience when she was called up, but said she 'just got on with the job' Mrs Cartwright said while the tunnels were built to protect production lines from bombing, by the time she began working at Drakelow the worst of the raids had already hit the country. Right, a newspaper cutting from 1941 detailing the deaths of three men killed when one of the tunnels collapsed during 'excavating operations' 'I'd had no experience at the time, someone, I think my sister-in-law, just gave me a book which told me how to do it and I just got on with it,' Mrs Cartwright told MailOnline. 'The metal could come in and it would be one of the men who would drill it into shavings. I'd dissolve it in acid and then put something on it that would allow me to analyse it so I could find how much nickle, chrome etc was in each part.' The only woman in the laboratory, Mrs Cartwright had wanted to serve in the army before being called up. 'In a way it was luck of the draw an din a way it wasn't. I wanted to go into the forces but parents in those days told you what to do and mine said: "no, you'll go into industry."' Earning £3 a week, she was also tasked with taking photographs of the metal for developing in a dark room next door. While the factory's design was so carried out to avoid raids, the worst of the bombing across Britain had already occurred by the time Mrs Cartwright took up work, she said. 'They started building the tunnels in 1941, quite late in the war. Around that time we were being badly bombed but never when I was there.' The site was restored in the 1980s by the Ministry of Defence which installed two KW generators . Tunnel 4, one of the main passageways used during the Second World War to shuttle workers to their stations . After the war ended, Mrs Cartwright trained as a teacher and spent years teaching P.E. in Worcester. She met her late husband, Cecil Cartwright, after the war had ended and the pair married in 1953. Revisiting the Drakelow Tunnels with one of her sons, John, the great-grandmother said it was 'strange' to see it so quiet. In 1936, the Air Ministry approached the car manufacturer Rover, asking them to help with wartime production. By 1939, all motoring production had ceased and the company had begun working solely on aircraft parts and engines. But when its Coventry factory was blitzed, bosses cited the need for a more secure setting to allow production to continue. Shadow Factories were then created. Built underground, safe from the shelling of Hitler's army, workers were free to work on engines for military aircraft and tanks. A canteen that was reused  during the Cold War when Drakelow Factory became a Government bunker . The Drakelow site was one of the largest, spanning more than three miles. Built into Kingsford Country Park, the factory is 300ft from the top of the hill but is in fact at road level. It was used to produce aircraft engines specifically for four models of Bristol aircraft; the Pegasus, Hercules, Centaur and Mercury. When demand wavered at the end of the war some staff were kept on to continue working on tanks. Production of the Meteor Tank Engine continued until 1952 when parts became surplus to requirements. All production stopped indefinitely thee years later when the site was handed over to the Ministry of Works. In 1961 the Government converted half of the tunnels to become a nuclear-safe bunker known as Regional Seat of Government. By the 1980s it had been modernised again with new blast doors fitted to meet updated requirements. It was decommissioned and sold to a private company in the early 1990s and remains in their ownership. 'All my memories are of a bustling hive of activity. I did enjoy my time there, it was certainly an experience. I just got on with it.' The Drakelow Tunnels or Shadow Factory was one of 26 of the same kind across the UK, built into the ground or landscape. During the war it was used to produce engine parts for four Bristol aircraft - Hercules, Pegasus, Centaur and Mercury. For a short while afterward the war production continued but to create parts for army tanks. The site is owned by a private Swiss company, though historians are eager to turn it into a permanent museum.
Mary Cartwright worked at Drakelow Underground Factory from 1943-1945 . Aged 20, she analysed metal in its laboratory for Bristol Hercules engines . Now 91, the great grandmother has revisited the site in Kidderminster . It was one of 26 underground factories used to produce military parts .