Lake Piso (also written Lake Pisu) is a large brackish coastal lagoon that lies behind Robertsport, separated from the Atlantic by a narrow strip of sand. It is the largest lake in Liberia and one of the country's most important wetlands — designated a Multiple Sustainable Use Reserve, with a Ramsar listing under the wetlands convention.
The lake is shallow, warm, and excellent for paddling — pirogue trips arranged from the Robertsport waterfront take visitors across to forested islands and to small Vai fishing villages on the far shore. Birdlife is excellent: herons, kingfishers, fish eagles, terns and seasonal migrants from Europe. Manatees are also said to inhabit the deeper sections, though sightings are rare.