Are there Seas on the Moon?

Even a quick look at the moon's surface reveals light and dark areas. The lighter surfaces are known as 'terrae' (from the Latin for earth), and the darker surfaces are known as 'maria' (from the Latin for sea). Early astronomers believed that the latter were oceans. Although later on astronomers realised that there wasn't any water on the moon, the name remained. The lighter areas, or the so-called continents, are also commonly referred to as lunar highlands due to their higher altitude.
Módulo del Jardín de Astronomía que simula la luna para poder observar su superficie The distribution of these two surface types differs from one face of the Moon to the other. Whereas nearly one third of the near side of the Moon is covered by 'seas', these represent only 2% of the surface of the far side of the Moon. But how did these physical structures form?

Approximately 3.8 billion years ago, the Moon's crust was heavily bombarded by asteroids, creating numerous impact craters with diameters hundreds of kilometres wide. These craters were subsequently filled by lava, forming oceans of molten rock. As they cooled, they formed the 'seas' that we see today. Craters account for the majority of the Moon's surface features. In fact, there are approximately 30,000 craters with a diameter greater than 1 kilometre on the near side of moon, and countless more that are smaller.