Son Doong Cave tours booked out until end 2016 after US TV appearance

  

Oxalis Travel Co, the only tour company licensed to conduct trips to the cave, said some 300 people had been Son Doong since it was opened to visitors in August 2013.

Son Doong is in the Unesco-recognised Phong Nha-Ke Bang National Park, Quang Binh Province, near the Laos–Vietnam border. It is biggest known cave in the world. Inside is a fast-flowing subterranean river and is so large it has its own ecosystem.

The cave was found by a local man, Ho Khanh, in 1991,  but it was not properly explored until 2009, by a group of scientists from the British Cave Research Association, led by Howard and Deb Limbert. The cave is 150m wide, 200m high and more than 5km long.

In October 2014, local authorities allowed a Vietnamese company to survey the cave for building a 10km suspension cable to transport visitors, but work was suspended following public opposition. There were concerns that large numbers of tourists might damage the cave's ecosystem.

On May 13, the US television network ABC broadcast live from the cave on its "Good Morning America" programme to its six million viewers using drone camera to show off the enormity of the cave and its natural beauty. Deputy Prime Minister Vu Duc Dam appeared on the show as a special guest.