Question

Large, flat areas on the Moon's surface once thought to be remnants of ancient seas

Answer 1

These dark areas are called maria. They are thought to be formed by volcanic eruptions in the moon's history.hope this helped:)

Answer 2

Answer:

Mare

Explanation:

Maria are large flat areas on the surface of moon formed by lava originated from volcanic eruptions millions of years ago. Earlier, these areas were thought to be as  large oceans and seas and hence given the name Mare which in Latin means sea. These are not very reflective areas and thus, appear dark.