Snaking for 10km beneath the cloud-swathed mountain range that separates Thailand and Myanmar is Tham Luang, the fourth biggest Thai cave system.
Its full name is Tham Luang Khun Nam Nang Non - "the great cave and water source of the sleeping lady mountain". Rich in folklore, it is a popular destination for tourists. It has its dangers - people have gone missing in Tham Luang before. And once monsoon season starts in July, the cave goes from innocuous to extremely dangerous.
The cave can flood up to 5m during the rainy season, and should only be entered between November and April.