Plural of 'mare,' a female horse; also 'maria' in Latin astronomical terminology refers to dark regions on the moon.
From Old English 'mere,' and Latin 'mare' (which means 'sea'). In moon terminology, early astronomers thought dark regions were seas, so they borrowed the Latin word 'mare,' making the plural 'maria.'
When astronomers looked at the moon, they thought dark patches were oceans and called them 'maria'—they were completely wrong about what they were, but the name stuck for 400 years!
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