The landscape of Wallonia, like Swiss cheese, is riddled with holes.
There are literally hundreds of caves and grottoes, with two 'caving schools' for learners in Namur, and two more near Rochefort and Liège, where groups of up to a dozen people can explore the subterranean wonders, including some mysterious lakes, as long as they're accompanied by at least two instructors. Tours last between 3-4 hours.
Caving is a hazardous activity, and no-one is allowed below ground without the proper equipment (helmet with lamp, harness and cave suit), which are all available for hire.
Insurance must also be purchased in advance.
There are many more options for experienced cavers who have international accreditation.
UK caving club members are especially welcome, because the Wallonian Caving Society (www.speleo.be) is keen to develop reciprocal ties which would give their 1,000 members easier access to British caves.