Over the last months, I enjoyed the 'discussion' below among the visitors about how to get to the mines in Spiennes (and even more important - how to get in). But gradually the basic visitor information became clear: there are guided tours on the first Sunday every month, and you have to be very alert in the town of Spiennes to spot the site. To make it even easier: via the link 'Mining site of Spiennes' above there's access to a map.
Besides an extra round on the Mons Ring road (there's only a small sign to Beaumont), I had no trouble finding it. In Spiennes the 'minieres' are even signposted. From above the ground, the site looks remarkable like any farming field. The archeological excavations are limited to two areas: one official site and one for volunteers.
One of the guides showed me around. First some open excavations on the surface. After that we descended a steep iron ladder, to get 10 meters below the ground. Here we arrived in a well-lit cave network (the electrical lights were only recently installed). The difference between the white chalk and the 2 flint layers was clearly visible. The guide explained the techniques used by the neolithic people to get to the flint. It was a tough and patient job carried out precisely. An amazing experience to stand here thousands of years later.