Mammoth Cave is the most accessible of the major show caves near Margaret River, with a self-guided audio tour and boardwalks through broad chambers of limestone. It is less theatrical than Lake Cave but easier to fit into a flexible day, especially if you do not want to commit to a timed guided tour.
The cave sits in marri and karri forest south of town, close enough to combine with Boranup Forest or a coastal drive. The fossil displays are the hook: bones from extinct megafauna were found here, giving the cave a scientific weight that goes beyond pretty stalactites.