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Mare Tranquillitatis

[trang-kwil-i-tah-tis]
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noun
  1. (Sea of Tranquillity) a dark plain in the first quadrant of the face of the moon: about 110,000 sq. mi. (285,000 sq. km).
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018