Hospital Cave is a three-level secret hospital and bomb shelter carved into a karst hill on the main road across Cat Ba Island, used by the Viet Cong from 1965 to 1975. It is one of the more impressive war-era sites in northern Vietnam, with around 17 rooms hollowed out behind a thick concrete and steel door that could be sealed against bombing.
A small entrance fee includes a guide — well worth it, as the cave is otherwise unsigned and atmospherically empty. Among the rooms you can see the surgery, the maternity ward, the small cinema, and the senior officers' quarters. Bring a torch even with the lights on; some side galleries are dark. It is roughly halfway between Cat Ba Town and the national park headquarters, an easy stop on a hire scooter or as part of a park-and-cave day.