Hospital Cave served both as a secret bomb-proof hospital during the American War and as a safe house for Viet Cong (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.
The cave spans 17 rooms, including an old operating theatre (complete with patient mannequins) and a huge natural cavern that was used as a cinema (and even had its own small swimming pool).