, is indeed one of the most spectacular geological wonders of the world. It is the largest intact and unfilled volcanic caldera on the planet. Formed about 2 to 3 million years ago when a large volcano exploded and collapsed in on itself, the caldera measures about 20 kilometres (12 miles) in diameter, covers an area of about 264 square kilometres (102 square miles), and has a depth of about 610 meters (2,000 feet).