About 4km from Ban Gioc Waterfall, 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 excellent lighting have been installed.
Frequent buses link Cao Bang and the road that leads to the cave, a trip of two hours (80,000d, hourly from 5.30am to 6pm).
A second, even bigger branch of the cave system is said to extend almost all the way back to Ban Gioc Waterfall, though there’s currently no visitor access to this section.