About 4km from Ban Gioc Waterfall, 2km up a side valley, Nguom Ngao Cave is one of the most spectacular cave systems in Vietnam. Created by an underground river, it extends for several kilometres underground; villagers sheltered here during the 1979 war with China. Visitors are permitted in one section, where a 1km-long concrete path and lighting have been installed.
Buses between Cao Bang and Ban Gioc can drop you at the valley turn-off, where you can look for a xe om (motorbike taxi) for the remaining 2km.
For an overnight stay here, head to one of the eight rustic homestays (from 100,000d per person, plus 100,000d for dinner) in the charming stone village of Khoi Ky, 1.5km before the cave.