High-resolution altimetric data define the detailed topography of the northern lowlands of Mars, and a range of data is consistent with the hypothesis that a lowland-encircling geologic contact represents the ancient shoreline of a large standing body of water present in middle Mars history. The contact altitude is close to an equipotential line, the topography is smoother at all scales below the contact than above it, the volume enclosed by this contact is within the range of estimates of available water on Mars, and a series of extensive terraces parallel the contact in many places.