Hammar Lacus is one of a number of hydrocarbon seas and lakes found on Saturn's largest moon, Titan. Its area is given as 18600 km2. (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.