This week we are travelling across the ocean to Vietnam to see the world's largest cave. Hang Son Doong (Mountian River Cave), the largest cave in the world, is located right in the heart of Phong Nha-Ke Bang National Park in the Quang Binh province of Vietnam.it stretches for over three miles (five kilometers), and it reaches heights of 650 feet (200 meters). The main cavern is large enough to house an entire large city block.