Langsir Cave is located on the southwest of Peluru Strait, facing the Langgun Island. The cave exhibits various cave features, developed within the Lower Limestone of the Setul Formation. The limestone is very rich in fossils and one can easily find fossils at this site.
The lake is a doline filled by saline water and surrounded by vertical limestone cliff. Ancient encrusting oysters and barnacles are abundant on the cave roof at Langsir Cave, some 2 to 3 meters above the present sea level.