Introduction to Maastricht Underground: Maastricht Underground originated from the Cretaceous period about 7 million years ago. The precious mosasaurus fossil and Delopodon fossil were unearthed in the cave. . The passages in each cave are complicated, with the largest number being more than 20,000. In order to prevent the roof of the cave from collapsing, many large pillars were left as supports during the tunneling.