Galileo first named the dark areas of the Moon mares (or maria for plural) after the Latin word mare, which means “sea.”
They looked like large oceans, and that’s what they were thought to be until the twentieth century.
We now know that mares are large, relatively flat plains of lava hardened into basalt.
Oddly enough, they are connected in much the same way Earth’s oceans are, but that’s as far as the similarity goes.
They carry such fancy names as the Sea of Tranquility, the Lake of Dreamers, the Bay of Rainbows, and the Marsh of Decay.
The names have no scientific or descriptive meaning.
A lunar mare appears darker than the “highlands” because of their iron-rich compositions.