/ Dictionary / Index M Mare Undarum: Meaning and Definition of Find definitions for: Ma're Un•da'rum Pronunciation: (un-där'um), [key] (Sea of Waves) a dark plain in the first quadrant of the face of the moon: about 4800 sq. mi. (12,000 sq. km). Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. Mare Tyrrhenum Mare Vaporum Related Content Daily Word Quiz: cautious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language