Mare Humboldtianum /hʌmboʊltiˈeɪnəm/ (Latin humboldtiānum, the "Sea of Alexander von Humboldt") is a lunar mare located just to the east of Mare Frigoris, in the center of Humboldtianum basin. It is located along the northeastern limb of the Moon, and continues on to the far side. Due to its location, the visibility of this feature can be affected by libration, and on occasion it can be hidden from view from Earth. At the center of the Humboldtianum basin is a mascon, or gravitational high. The mascon was first identified by Doppler tracking of the Lunar Prospector spacecraft.