The Zonneberg is one of the more interesting cave systems near Maastricht. It is located on the southern outskirts of the town, on the far side of the 110 meter high St Pietersberg hill. The cave systems have been gradually created over time as sandstone has been quarried out of the hillside. There were more than 20,000 passages, but only a few thousand are still accessible today. Some of them were used as air raid shelters during World War II. Visitors can explore the caves with the aid of a local guide.