The caves of Postojna in Slovenia are the second largest caves in the world that are open for tourists and one of the few caves that are wheelchair accessible. A small train runs through the underground vaults, through tunnels and huge halls with various karst formations.
The train is narrow , but the last compartment has no seat and is large enough for a wheelchair. Even paths lead further into the caves– not a single step, but every now and then extremely steep and slippery. It is worth it though– very impressive! Link to the cave: Postojna Cave