maria is the plural form. mare is the singular form. (pronounced mah-ray, not like the horse)
you gotta know your latin if you wanna sound smart in astronomy.
Transcripts
sedna: the dark parts of the moon are called “maria”, meaning “seas”, because early astronomers thought they were actual seas.
sedna: …but then it turned out they are just dark plains of rock.
sedna: …but then it turned out they used to be seas of lava.
sedna: sometimes science ruins things but then makes them even cooler.