The next time you see the full moon high in the south, look carefully at its
right-hand edge and let your eye travel upward along the curve of the disk.
Round about two o'clock you will notice a small, dark oval: anyone with normal
eyesight can find it quite easily. It is the great walled plain, one of the
finest on the Moon, known as the Mare Crisium---the Sea of Crises. Three hundred
miles in diameter, and almost completely surrounded by a ring of magnificent
mountains, it had never been explored until we entered it in the late summer of
1996.
[extract from SENTINEL OF ETERNITY (1951) by Arthur C. Clarke]