These underground caves, 2km northwest of Fátima, are part of the surrounding limestone massif. They were discovered in 1971 when two hunters chased a fox that disappeared into a hole. Visitors can enter the caves (which are atmospherically lit and are an extraordinary 18°C), descending via steps to a depth of 45m on a 350m circuit. It won't ping the 'wow, the best thing ever' scale, though it's interesting. Tours are fully guided (English, German and French guides available; 35 minutes).
You also have an option to visit the Centro de Interpretação on the same premises. It highlights the geography and geology of the region, explaining why its full of minerals and fossils. One room has minerals from all around the world, and (of course) there's the ubiquitous souvenir shop.