Trung Trang Cave is one of the larger limestone caves on Cat Ba, about a kilometre long and threaded through a karst ridge near the centre of the island. It is rich in stalactites and stalagmites and lit, slightly garishly, with coloured floodlights — Vietnamese cave aesthetic — but the formations themselves are genuinely impressive.
The cave is on the main road across the island, just before the national park headquarters when driving from town. Tickets are sold at the small kiosk by the road; a local guide takes you through in about 30 minutes. The path is paved but slippery in places, with low ceilings in spots — watch your head. Combine with a national park hike on the same trip out from town.