Here's why:

* Maria are large, dark plains on the moon's surface. They are made of solidified basalt, a type of volcanic rock.

* Volcanic eruptions: These plains were formed by ancient volcanic eruptions that spewed lava onto the moon's surface. The lava flowed across the lunar landscape, filling low-lying areas and creating these smooth, dark plains.

* Impact craters: While some impact craters also contribute to the formation of maria, the vast majority of these plains are directly linked to volcanic activity.

So, the presence of maria on the moon is a key piece of evidence that the moon once had a molten interior capable of driving volcanic eruptions.