The area of the southern Limburg district is very rich, a valuable rock is found underground: limestone. This seems a bit strange, as limestone is a rather common and cheap rock nowadays. In former times, when transport was rather difficult and expensive, so the existance of those quarries was rather important for the Netherlands. The limestone was needed for building, mortar, the garden and to make white colour.
There are maybe thousands of limestone quarries and other underground caverns. They are not connection, but the were built because of similar reasons, which was the search for limestone.
Several of those caverns are open to visitors. They are not listed seperately beacause of the lack of information. Most of them are either not accessible or only after talking to the owners, which is a bit difficult for foreign tourists, especially if they do not understand the language.
The website by Ed Stevenhagen, listed below is a very detailed and informative. Unfortunately most of it is in Dutch, which I do not understand.