The limestone caves in Halong Bay provided refuge for the north Vietnamese during the Vietnam War. A tour will take you to several of these caves, including one that had been converted into a hospital, and another that held prisoners of war. Neither cave appears as it did at that time, there are no artifacts or war relics to see.
In this excerpt, the tour guide is in the hospital cave, discussing the number of bombs that were dropped in north Vietnam during the roughly 13 years of American involvement in the war. Civilan and military casualty numbers vary. The tour guide mentions 4 million Vietnamese casualties, which is a very high number, compared to other sources.
This oozes historical significance, as it 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. 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 which was used as a cinema (and even had its own small swimming pool). The cave is about 10km north of Cat Ba Town on the road to the national park entrance.