Hang Son Doong is a cave in Vietnam, located near the Laos–Vietnam border, that is currently the biggest known cave in the world. Inside is a vast green forest and a large, fast-flowing underground river.  The biggest chamber of Hang Son Doong is more than 5 kilometres long, 200 metres high and 150 metres wide. The cave’s enormous dimensions has allowed stalagmites to grow up to 70 meters tall, the tallest known stalagmites in the world. Cave pearls found inside the cave were the size of baseballs, an abnormally large size. Despite its enormity the cave was only discovered in 1991 by a local man, but the local jungle men were afraid to explore because of the whistling sound the underground river made. It was not until 2009, when a group of British scientists from the British Cave Research Association conducted a survey, that the existence of the cave was made known to the public. Later, a team of explorers from National Geographic came up with some of the most stunning pictures of this previously unseen natural wonder.