Hospital Cave served both as a secret, bomb-proof hospital during the American War and as a safe house for VC leaders. Built between 1963 and 1965 (with assistance from China), this incredibly well-constructed three-storey feat of engineering was in constant use until 1975. The cave is about 10km north of Cat Ba Town on the road to Cat Ba National Park entrance.
A guide (most know a few words of English) will show you around the 17 rooms, point out the old operating theatre and take you to the huge natural cavern that was used as a cinema (and even had its own small swimming pool).
