Just visited - April, 2015. Only one location - Bois du Cazier - relatively easy to see if you are in Charleroi (in fact Bois du Cazier is currently part of the city).

There is a public bus to go there (# 52) starting from Charleroi-Sud, but it is not very frequent (on Sundays there are only 3 buses to go to the mine). It is better to use bus # 1 - ask the driver to drop you in the place nearest the mine - around 2 km by foot or take any bus to Marcinelle (more frequent) and take some walk from there (3-4 km, check on the map, where to go).

The site itself is well organised, prepared for mass visiting (Industry Museum, Glass Museum, expositions) with special hall devoted to the biggest catastrophe in the history of Belgian mining.

Buildings covers relatively small area, sadly no underground part is allowed for visiting. Depending on your willingness to see exhibitions half a day should be enough for the whole trip