My wife and I visited Spiennes in August 2017. Contrary to the experience of other reviewers, we showed up randomly and managed to grab a free spot on the tour! It had just started so I missed the first few minutes, but it was in French and my grasp on that language is tenuous at best, so I didn't really miss anything. The guide was kind enough to give me the gist of what she was saying too.

The mines were surprisingly large underground, and I really enjoyed seeing the pits. Also very startled to find that in the hundred or so acres of the site, there are thousands of mines but only a handful have actually been excavated! Makes you really wonder what else is yet to be discovered.

See below for my full video review!