Planina Cave, 12km to the northeast of Postojna Cave, is the largest water cave in Slovenia and a treasure-trove of fauna (including Proteus anguinus). The cave’s entrance is at the foot of a 100m rock wall. It’s 6.5km long, and you are able to visit about 900m of it in an hour. There are no lights, so take a torch.
With prior warning, tours can be arranged most days in July and August – including a longer, five-hour tour with a boat (€30).