Mare Erythraeum is a very large dark dusky region of Mars that can be viewed by even a small telescope. The name comes from the Latin for the Erythraean Sea, because it was originally thought to be a large sea of liquid water. It was included in Percival Lowell's 1895 map of Mars.
Mare erythraeum iii
References
Mare Erythraeum Wikipedia(Text) CC BY-SA