Mare Undarum (the "sea of waves") is an uneven lunar mare located just north of Mare Spumans on the lunar near side, between the crater Firmicus and the eastern limb. It is one of the many elevated lakes contained in the Crisium basin, surrounding Mare Crisium. The selenographic coordinates of this mare are 6.8° N, 68.4° E. It has a maximum diameter of 243 km.