Don't miss these mysterious caverns that tunnel into the heart of high cliffs. The Romans and all who came after them took from Mount St. Peter great chunks of marlstone, a type of limestone that's as soft to carve as soap and hardens when exposed to air. Many Maastricht buildings are constructed of marlstone. As more and more marlstone was extracted over the centuries, the Sint-Pietersberg interior became honeycombed with 20,000 passages. From Roman times, to medieval sieges, to the 4 years of Nazi occupation during World War II, these chambers have served as a place of refuge. Many drawings and signatures have been left on the marlstone walls. During World War II, the caves sheltered from the Nazis Dutch masterpieces such as Rembrandt's The Night Watch and other treasures. You follow your guide's lantern through a labyrinth of 6- to 12m-high (20- to 40-ft.) tunnels. Stay close to that lantern -- tales are told of those who entered the 200km (120 miles) of tunnels and were never seen again (ask about the four monks). The temperature underground is about 50°F (10°C), and it's damp, so bring a cardigan or a coat.