Jenolan Caves, Australia

Jenolan Caves, at 340 million years old, rank among Earth's oldest open caves, featuring pristine crystal formations and an underground river system in New South Wales. The Lucas Cave chamber measures 90 meters in length with stalagmites reaching 15 meters tall, and the cave hosts 250,000 visitors yearly. Its perpetually moist 12°C climate ensures crystal growth continues, solidifying its status as a living geological marvel.