The Jenolan Caves in the Blue Mountains in New South Wales are the oldest discovered open caves in the world. They date back to 340 million years! Petroleum companies discovered this while finding oil deposits using a dating method.
The CSIRO’s petroleum resources division conducted the potassium argon dating on clay samples from the caves. The caves were previously estimated to be older than the 90 to 100 million years, but the caves being found to be dating back to the Carboniferous age surprised all.