The vertical shaft, which collapsed as FourWinds says, was actually cut in the late 1700s by the Duke of Northumberland.

It was left with a timber cover with chalk or soil over it, and one day the cover caved in.

There was also a side entry shaft in the mid 1800s, and one in the late 1900s for the Beeb.

This from memory - shoot me down if I'm wrong.