Early astronomers called dark plains “maria” (Latin for seas). They formed from basaltic lava flows. Covering about 16% of the surface, maria are mostly on the near side. They’re smoother than highlands and reflect less light. The name stuck even though no water exists. These features reveal ancient volcanic activity. The contrast between dark maria and bright highlands is visible from Earth.
It matters because maria dominate lunar landscape and affect observation. They provide landing targets and geological insight.
It also preserves history. Dark basalts are younger than highlands, revealing Moon’s evolution.
Maria can stretch over 1,000 km across.
NASA [nasa.gov]