Complex sentence: Oceanus Procellarum is the largest of the lunar maria, stretching more than across its north-south axis and covering roughly but is, nevertheless, still smaller than the surface area of the Mediterranean Sea on Earth.
Split into simpler sentences: Oceanus Procellarum is the largest of the maria, stretching more than across its north-south axis and covering roughly. Nevertheless, it is still smaller than the surface area of the Mediterranean Sea on Earth.