A tourist takes a photo while the sun shines deep into Son Doong cave through the first sinkhole (doline). Hang Son Doong, the world's largest cave, is located in Quang Binh Province, Vietnam. It was discovered in 1990 and was put into operation in the form of adventure tourism in 2013. Each year a maximum of 1,000 people can visit the cave, with 10 people on each trip. The journey to explore this cave takes four days and three nights. In addition to physical preparation, visitors need to plan to completely lose contact with the outside world because there is no phone signal or internet.