Harford Hills Chalk Mine
Norwich was mined for chalk and flint from the middle ages until the start of World War 2. The Harford mine was the last to close.
The entrance is deep within Danby Wood. The main tunnel is about 50m long.
Norwich City Councils website was the source of some of my information, it goes into more depth about Norwich’s mining history http://www.norwich.gov.uk/services/Pages/ChalkWorkings.aspx
There is some interesting information on the geology of Norwich here: http://www.heritagecity.org/research-centre/subterranean-norwich/geological-norwich.htm
Norfolk Uncovered made a good video about the mine, they even created a map of the tunnels. https://www.youtube.com/watch?v=hneVwuDdu-E
Its seems to have been a bit of a tourist attraction recently, though last I heard the entrance had been buried.
Below is a really bad video I made of my first venture into the mine.