Featuring its' own underground river, the Hang Son Soong in Vietnam is the largest known cave in the world and was discovered only recently in 1991 by a local man named Ho Khanh. It wasn't internationally known until 2009 when members from the British Cave Research Association, led by Howard and Deb Limbert, conducted a survey there. The ever-expanding cave was created 2-5 million years ago by erosion caused by river water flowing through limestone mountains.