Natural caves don’t come much larger than Sơn Đoòng, close to the border between Laos and Vietnam. This mammoth abyss has the largest cross-section of any known cave on the planet, a vast area that is difficult to describe. Supposedly, a Boeing 747 could fly through without damaging its wings, but that doesn’t really do justice to the vastness of Sơn Đoòng. The stalagmites here are pretty massive too, with some reaching up to a whopping 70 metres.