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A wide and flat basin, approximately circular in shape and some 350 kilometres across, that lies at the southeastern extent of the vast Oceanus Procellarum. The borderline between the Ocean of Storms and this sea is not distinct, and indeed historically this area was not considered a separate region. In 1964, however, the lunar probe Ranger 7 landed here and transmitted the first high-quality images from the surface of the Moon back to Earth. Thus this plain acquired the name Mare Cognitum, the 'Known Sea' or the 'Sea That Has Become Known'.
Though the boundary between Mare Cognitum and Oceanus Procellarum is not always clear, there is one feature that distinctly separates the two areas: a brief range of mountains, a remnant of the Sea's ancient northwestern wall, known as Montes Riphaeus. To the Known Sea's northeast lies a complex of walled plains, dominated by Fra Mauro, nearly a hundred kilometres from edge to edge. Along its southern bounds, Mare Cognitum is bordered by two similar small seas: to the southwest lies Mare Humorum, the Sea of Moisture, while to the south and southeast lies Mare Nubium, the Sea of Clouds.
The surface of Mare Cognitum is speckled by small craters and pits, but few of these reach significant size. The sole exception is the crater Darney in the southern part of the sea. Though still relatively small at just fifteen kilometres across, it is nonetheless the largest feature of note within the borders of Mare Cognitum.
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