The cave was found by local farmer Mr. Ho Khanh in 1991. It was not until 2009 that the British Cave Research Association explored the cave and it instantly became one of those amazing things you have to see in your life.In 2014, once the caves had been thoroughly explored, the government granted tour operators permission to host treks through the caves. Since then, approximately 900 licences have been awarded!
ref source: https://urszihlmann.com/hang-son-doong/