This former medical centre would have been used to treat injured workers and although not quite underground its buried behind a large wall with a small access space and under a car-park; it's sadly now in a serious state of decay, however still worthy of a visit.
Firth Brown Steels was initially formed in 1902, when Sheffield steelmakers John Brown and Company exchanged shares and came to a working agreement with neighbouring company Thomas Firth & Sons. In 1908 the two companies came together and established the Brown Firth Research Laboratories and it was here, in 1912, under the leadership of Harry Brearley they developed high chrome stainless steel. The companies continued under their own management until they formally merged in 1930 becoming Firth Brown Steels. The company was amalgamated into Sheffield Forgemasters in 1982.