Opened to the public in 1940, this limestone cave with surreal, gleaming calcite formations remains one of Australia's finest show caves. It can only be visited as part of a guided tour, which takes 45 minutes and is totally illuminated, with clear walkways. The interpretative centre has good info on the cave's history. It's within Lower Glenelg National Park, 17km north of Nelson, and crosses the border before looping back over; it's best to get here via Nelson River Cruises.
There's a campground here, too, along with numerous walking trails.