Any particular part of the Durham Coalfield? It's a fairly large one.
Some of the coastal shafts/concealed parts of the coalfield will be deep enough and were generally sunk late 1800s / early 1900s, so 1906 is in the earlier years (or during shaft sinking) for the coastal ones. I'm not sure of the deepest but Wearmouth was >1900ft deep
gNick Oddly I tried my link to DMM, but I appeared to have an old broken link. I've just gone back to www.dmm.org.uk and it worked fine.
The index to the mines (which includes the depths and other information):
http://www.dmm.org.uk/colliery/index.htm
I went in a mine once.... it was dark and scary..... full of weirdos
When do I get my soapbox, I need to rant on about some b***cks