Sunday I was diving in an old black marble mine in Belgium. The mine is not really big. Depth was now 30m, but depends on the ground water level. The viz in the open water part is almost zero, but when you swim in the overhead part, at around 18-20m depth, the viz clears up. You can swim the mine around in about 20 minutes. There is 1 place where you can lay a jump to another short tunnel. It is not a real full cave dive, but to swim the mine around you pass several T's. But it is nice to dive in this mine as it is not too far from the Netherlands. The mines in Germany that are open for divers are in Northrhein Westfalen bigger, but more expensive. That are all slate mines (the only iron mine is sadly closed for diving now). Here it is a marble mine.
The mine is open for full cave divers and the owner is friendly. He tries to improve parking lots, sells some cheeceburgers, etc. You pay 7.50 euro for 2 dives.
I took my camera and took some pictures. As the walls are mostly black, most light from strobes is absorbed. But I am happy with the resuls. It gives an good impression of the mine.