Dad and I took a boat ride down the Maas River to St. Pietersberg, a few miles south of town. St. Pietersberg is a large hill (which the Dutch call a "mountain") that has been mined for limestone since Roman times. We toured part of the mine inside the mountain, walking through galleries that had been mined for centuries. During World War II, the city of Maastricht established an evacuation center in this old mine that could have held up to 50,000 people. Ultimately, about 5,000 people were sheltered there for a couple of weeks.