Neagh Lacus is one of a number of hydrocarbon lakes found on Saturn's largest moon, Titan. The lake is composed of liquid methane and ethane, and was detected by the Cassini space probe. (wikipedia entry)
A lacus is a "lake" or small, dark plain with discrete, sharp boundaries. On Titan they are named after lakes on Earth, preferably with a shape similar to the lacus or lacuna on Titan.
Image Source. Marker locations are approximate and may not always appear to align with image on map.