One of the most impressive sites near the terminator of the 21-day old moon is Mare Imbrium, a large circular formation lying in the northwestern quadrant and whose diameter of 1250 kilometers and area of approximately 830,000 sq meters make it the second largest mare on the moon. It is believed to have been formed between 3.80 and 3.85 billion years ago and is characterized with a very flat and smooth floor with some "wrinkling" on the periphery. Some dominant features of interest include the large crater Plato on the northern rim, the appenine mountain range to the east and the three large craters Aristillus, Autolycus and Archimedes immediately west of the appenines. Careful examination of the floor indicates a differential color pattern between the northern portion and the rest of the area immediately south indicative of differential geological activity.