You can explore most Slovenian caves on foot, but Križna (Cross) Cave, one of the most magnificent water caves in the world, is one of few where you can take a subterranean boat ride. This is also Slovenia's only tourist cave without electric lighting – visitors are given lamps (and boots) for their visit, which lasts one to 1½ hours and tours the dry part of the cave, including a short boat ride at the first lake. Book ahead.
Križna is 8.8km long and counts 22 underground lakes filled with green and blue water. It's well known for the amount of fossilised cave-bear bones found inside.
With advance booking, you go as far as the Kalvarija chamber by rubber raft via 13 lakes (per person €45 to €65, depending on group size). It's a physical, four-hour tour, and the price includes all equipment. Visitor numbers are restricted to about 1000 annually, so it's a good idea to book ahead. In winter (October to March), there is a seven-hour tour, too (per group €240; restricted to about 100 per year).
The cave is signposted off the main road about halfway between Cerknica (11km northwest) and Snežnik Castle (10km south).