Scientists believe that lunar maria formed from lava flows caused when the surface of the moon was disrupted by the impacts of gigantic meteors and comets. Maria is the plural of "mare," which is Latin for sea. One of the most famous of the lunar maria is the Sea of Tranquility, which was the site of the first moon landing.
If a large enough object struck the moon, it may have actually cracked the surface, causing magma to bubble up from the mantle and spread out into the surrounding moonscape and into the impact crater itself, then cooling into a volcanic rock called basalt. The objects that impacted the moon had to be very large. Some maria on the moon are over 621 miles across; scientists believe that it takes an object about one-tenth that size to create such a large feature.
Maria are often darker than the surrounding landscape, and the youngest ones are smooth and nearly devoid of impact craters. Meteors and other debris that struck the moon were common when the moon and the solar system were first forming, but as the solar system aged and became more settled, the number of impacts decreased. The rocks found in the lunar maria are estimated to be about 3.8 billion years old.