A vast labyrinth of man-made tunnels beneath the Sint Pietersberg hill on the southern edge of Maastricht. The caves were carved over centuries by miners extracting marlstone — the soft limestone used to build much of the city — leaving behind more than 20,000 passages stretching over 200 kilometres. The temperature inside is a constant 10°C and the darkness is absolute.
Guided tours lead you through a small section, passing centuries of charcoal graffiti, carved sculptures left by bored miners, and chambers where art treasures and refugees sheltered during wartime. The scale is extraordinary — this is one of the largest underground tunnel systems in the world, and most of it remains unexplored by visitors. Tours depart from the Caves Centre at the base of the hill; book ahead in summer. Bring a warm layer.