(information taken from Wikipedia)
The Ma On Shan Iron Mine opened in 1906 as an opencast site run by the Hong Kong Iron Mining Co. Ltd., which was owned by Sir Paul Chater. In 1949, the mine was taken over by the Mutual Mining and Trade Company, which extended it underground in 1953. By 1959, mining had moved entirely underground. The mine ceased operation in 1976.[The workforce of 400 was laid off. The government mining lease ended in 1981 and the mine closed subsequently.
Youtube video of inside the mine (courtesy of CX15): https://youtu.be/PXMHujhuz0E