A cave museum is a museum dedicated to cave science (speleology) in all its aspects, but there are also museums that focus on one aspect that is of great local importance, for example cave bears.
Speleology is a science that in a way lies at the intersection of a number of other sciences. It deals with caves, or the umbrella term karst, and utilizes all other scientific disciplines. Geology is just as important for the formation of caves as chemistry or even physics, while speleothems clearly belong to mineralogy. Cave animals are an important and interesting subgroup in biology, and the contents of caves are of great importance for archaeology and palaeontology. And this description of speleology at the intersection of various sciences is also a wonderful definition of a cave museum, also called a speleological museum. This type of museum is usually dedicated to a specific karst area, it shows the geology of the area with pictures and often with a cave replica. Other topics include speleothems, cave animals, the typical vegetation of the dry karst surface, the fossils in the rock and the archaeological remains in the cave.
We describe cave museums on showcaves.com because they are very closely related to our topic. Actually, they fall out of the definition: they are not underground. But their topic, their content, is underground, they are dedicated to caves, just like us. If they are dedicated to a sub-science, such as palaeontology or cave biology, the type of cave is irrelevant. However, if they are dedicated to cave formation itself, the subject is practically always the karst cave, i.e. the cave in limestone. There are very few that are dedicated to lava caves or gypsum caves, but that's it. The large number of other cave types, which are admittedly very rare, are not represented in museums.