Suisei Planitia is a large basin on Mercury. Ghost craters are unusual forms that occur in the Suisei Planitia. They are buried and rounded in profile, with only their rim crests rising above the surrounding smooth plains. It has been suggested that the smooth plains that Suisei Planitia is part of might be ejecta from Caloris that were melted by the impact. (wikipedia entry)
A planitia is a low plain. On Mercury they are named after names for Mercury (either the planet or the god) in various languages.
Image Source. Marker locations are approximate and may not always appear to align with image on map.