Caves of the Ardennes (Belgium). 'You don’t need to be a daring speleologist to explore some of northern Europe’s most awesome cave systems, hollowed out beneath the rolling countryside of the Belgian Ardennes. The best known at Hansur- Lesse even starts with a train ride, while at Remouchamps you float part of the way on an underground river.' http://www.lonelyplanet.com/belgium/wallonia/liege
When you're heading to Belgium, be sure to visit Liège, with its historical center on the one hand and vibrant university district on the other hand! View through stained glass window on inner garden of the Cathedrale St Paul (vue prise du Cloître)
Université du Val Benoît (Liege, Belgium) Situated on the banks of the Meuse, this former university once produced some of the finest mathematicians in Belgium. Empty since 2005, it now serves as a magnet for urban explorers, thanks to its sheer size and air of otherworldly decay.