Jenolan Caves
By measuring the ratio of radioactive potassium and trapped argon gas, which was produced when the potassium decayed, scientists determined the age of the clay in the caves to be approximately 340 million years old, thereby making the cave complex the world's oldest known and dated open cave system. The Commonwealth Scientific and Industrial Research Organisation (CSIRO) in association with the University of Sydney and the Australian Museum lead the efforts in scientific research into the caves.