Hippalus is the remnant of a lunar impact crater on the eastern edge of Mare Humorum.

The southwest rim of Hippalus is missing, and the crater forms a bay along the edge of the mare.

The surviving rim is worn and eroded, forming a low, circular mountain range.

The crater floor to the east of this rille is more rugged than the area in the western half.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Hippalus.