Some 2000 years ago when the Romans expanded their empire, they ventured as far north as Valkenburg in the southern tip of the Netherlands. They needed building materials, began excavating the hills and found soft sand stone which they used for building their villas. These excavated hills are now the Gemeentegrot or Municipal cave.
The caves can be visited on a guided tour. It is essential to stay with your guide as the cave consists of a warren of dark alleys. The guide will point out fossils and shells in the different layers of lime stone. During WW II the caves served as a hiding place and several murals commemerate this.
If walking is too strenuous for you, hop on the tourist train that trundles through some of the wider passages.