I have a map uploaded in the Cults folder that shows all of the entrances and their locations, but your comment prompted me to remove the coordinates.
The entrance you are looking for is in the east, and is the adit that served the modern grid-style workings.
The place is easy to find if you know what you are doing. Please know what you are doing... because if you don't you'll have a bad time. The place is literally a maze and some bits are death traps. I'm pretty sure the adit you seek ends in water and a collapse only a short distance inside. Also, most of the other entrances are collapsed or bulldozed.
Don't become a statistic.