Marbo Cave is a freshwater cavern in the northern part of Mangilao, off Route 15 near the Marbo Cliffs subdivision. A short trail descends from the road into a limestone hollow where a pool of clear, cool freshwater has formed at the bottom. The water is reputedly safe to swim in, but the steep entry and lack of any facilities mean it is mostly visited by local teenagers and the occasional curious traveller.
This is a low-key, "if you have time" stop rather than a destination in itself. Bring water-shoes, a flashlight, and a willingness to scramble. Do not attempt it alone or after heavy rain.