Mare Crisium is a prominent circular sea near the eastern limb of the Moon's visible face, foreshortened by its location into a narrow oval shape as seen from Earth. Surrounding the sea is a ring of higher land, and to the northeast of Mare Crisium, running approximately north to south through the highlands ringing the larger sea, is a narrow valley with a dark basalt floor.
This valley follows a gradual looping shape running for approximately 150km from end to end, and from this shape it takes its name of Mare Anguis, the Serpent Sea. It cuts through a region with few prominent craters, but one stands out. To the north and west of Mare Anguis is a crater named Eimmart whose diameter of 46km approaches one third of the length of the twisting Serpent Sea itself.