Australian-New Zealand couple Scottie and Nicki share a moment in the world's biggest cave. Hang Son Doong in central Vietnam near the Lao border is a remote place where visitors tend to bond with the extraordinary subterranean surroundings and with each other. The tiny figure on a massive stalagmite in the background is Howard Limbert, the original explorer of the cave, who introduced Hang Son Doong to the world.