Hang Sơn Đoòng Cave is located in Quang Binh province in Vietnam and was found as late as 1991 by a local. However, it was only opened to limited tourists visits since 2013. It is the world’s largest natural cave system and is estimated to be 2-5 million years old. Though already a massive cave, it was discovered in 2019 that Sơn Đoòng is connected to the Hang Thung cave through its underground river.