Rarely can the word “cavernous” have been used with such justification. In 2009, a group of British cavers attempted the first-ever detailed survey of the Son Doong cave, in Phong Nha-Ke Bang National Park, finally giving up 4.5km in. Their records and photographs showed chambers large enough to swallow up whole city blocks – the largest found so far is over 250m high, and 150m wide. Subsequent investigations have added another 2km to the cave’s charted length, and shown the presence of 70m-long stalactites, gigantic shards of crystal and grapefruit-sized calcite pearls. The cave is highly remote and, at the time of writing, had not yet been opened to the public, but it seems almost certain to become one of Vietnam’s most alluring sights.