Here's why:
* Maria (Latin for "seas") are large, dark, smooth plains on the Moon's surface. They are actually vast impact basins that were later filled with lava flows.
* The lava flowed from volcanic eruptions, which were active in the Moon's early history. These eruptions were likely triggered by impacts from asteroids and comets.
* The maria are darker than the surrounding highlands because the volcanic rock (basalt) is darker and smoother. This makes them appear like seas when viewed from Earth.
So, the presence of maria is a clear indication that the Moon once had a much more active geological history than it does today.
