Hi,
"Coffin" is a term used to describe the shape of a drive (tunnel); If you look at their profile they look just like a coffin hence the name.
I spent 3 great years at Camborne School of Mines and spent far too much time slipping off underground around Cape Cornwall.For my sins I was also member of the mine rescue team too long ago to count! I was fortunate to work at Wheal Jane and South Crofty as a student.
There are two excellent books that cover the St Just mining district in detail. I'll dig them out and post the ISBN, author and Title. Some of my collection of mining books come from the St Just United mine, I bought them in a junk shop in Camborne a lucky find!
I'd love to move back down their and spend my time mapping the workings.
Just be very careful, some of the shafts are DEEP and if you're on you're own you may never be found. If you go in the rough and are uncertain I would recommend belaying yourself and be prepared to climb out. Better safe than sorry.
Cheers,
Wes