Photo credit: CC license by Jojo
7. Hang Son Doong – Vietnam
Located in Vietnam the Hang Son Doong is the largest known cavern on Earth. It was discovered in 1991 because of the whistling sound that shot through its inner chambers and then shown to the rest of the world in 2009. Tours are offered to adventurous visitors that last upwards of 6 days inside of the gigantic cavern.