A short movie about an incredible journey through Hang Sơn Đoòng, the largest cave on Earth.
As of 2009 it has the largest known cave passage cross-section in the world, and is located near the Laos–Vietnam border. Inside is a large, fast-flowing subterranean river. It was formed in Carboniferous/Permian limestone and is believed to be between 2 and 5 million years old. [Source]