Deep under a limestone mountain in the tropical forest, you can follow an underground river through a series of vast grottoes in the Phong Nha Cave. Let a guide show you the spectacular system of subterranean scenery as you admire one of the great lesser-known wonders in Vietnam.

A ride from Hue in the morning carries you past Vietnam War battlefields and to the banks of the Ben Hai River. Get a look at the former Vietnamese Demilitarised Zone and the Hien Luong Bridge, and then stop for lunch at a local restaurant before you board a boat for a cruise that takes you inland to the mouth of the cave.

Looming at the bottom of a sheer cliff wall, the cave's angled entrance lets your boat slip into another world. Follow the water underground for more than half a mile (700 m) as the jagged stalactites stretch overhead, and then step onto solid ground to navigate a large, dry passage and venture even deeper in on foot. You can look at ancient altars and inscriptions left by the Cham people and get a sense of the immense depth of the cave system before you return to your hotel.