If you're not afraid of the dark, you should visit the famous Caves of St. Pieter. In the course of centuries of marl quarrying, an underground labyrinth was created here with over 20,000 passageways. The manual labourers who cut out the blocks as well as various artists have left their marks, including various interesting and sometimes very old writings on the wall. But the caves were used not only for quarrying marl. They turned out to be very handy hiding places for residents during the many sieges that Maastricht had to endure as well as World War II. Some of the facilities installed at the time are still present. The temperature in the caves is 9 to 10 degrees Celsius, so visitors would do well to take along a cardigan, sweater or jacket. The guided tours last about 1 hour and are supervised by official guides. Two different networks of tunnels have been opened to the public under St. Pietersberg hill.