Frog Lake is a freshwater karst lake hidden in the interior of Cat Ba Island, reached on foot from a small village called Gia Luan via a two-hour trail through national park forest. The lake itself is small and quiet — water lilies, occasional kingfishers, the occasional snake — but the walk to it is the better half of the experience, climbing through deep forest with views back over the valleys.
The lake is sometimes included in a longer through-hike from Gia Luan to Viet Hai village, with a boat back to Cat Ba town in the afternoon — about six hours of walking in total and one of the best ways to see the interior of the island. Take a guide; the trail is unmarked and the karst can swallow a navigation mistake quickly.