Near Broken River Bridge, 2km northeast of Castle Hill, you'll find this 594m-long cave. As indicated by the information panels in the car park, the walk through the cave is an achievable adventure, but only with a foolproof torch and warm clothing, and definitely only if the water level is less than waist deep where indicated. Heed all notices, take necessary precautions and revel in the spookiness.
Failing that, just wander around the 15-minute loop track for a gander at the surrounds, including limestone formations that featured in the Chronicles of Narnia movie.