What are lunar landforms were originally thought to be calm seas?

Lunar landforms that were originally thought to be calm seas are known as "maria," which is Latin for "seas." These dark, flat plains were formed by ancient volcanic activity and were mistaken for actual seas due to their smooth appearance and relatively low elevation compared to the surrounding highlands. Early astronomers, such as Galileo, observed these features through telescopes and believed they were bodies of water, leading to the misnomer. In reality, the maria are solidified basaltic lava flows.