The flat dark regions of the Moon were called “seas”(or maria, sing, mare) by seventeenth-century astronomers who thought they were the beds of ancient oceans(1). In that regard, they even thought of new life forms dwelling on the Moon, evolving in the deep ocean. But, what are the large dark areas on the Moon?
In the process of observation, it seemed incongruous that the Moon had ancient oceans at this scale leaving these dark regions due to its atmosphere state at that time. If the Moon had ancient oceans, they would vaporize before leaving any marks of their existence. Nevertheless, we now know that the dark regions on the Moon occurred as a result of meteorite impacts. The reason is as follows. Lunar craters range in size from a few millimeters to more than 250 km in diameter, with some few, called “basins”, attaining diameters of up to 2000 km(2). And, unlike the Earth craters, almost all lunar craters are non-volcanic in origin but as a result of meteorite impacts. Unlike Earth, the Moon does not have an atmosphere or geological activity to erase craters permanently on its surface. In other words, the Moon is geologically inert now and therefore looks much the same today as it did 3 billion years ago. So, the flat dark regions of the Moon is actually, without any doubt, the remains of the large-scale basaltic lava flows that filled the floors of indelible marks of craters on the Moon surface. These dark zones were named, according to their shapes and sizes, if the lava was confined inside the crater, the area appears circular, but if the lava flow spilt out through gaps in the sides of craters to cover adjacent craters, the area appears in a flat shape. And, lunar seas flooded by massive ancient lava flows look dark because of the blackish color of basalt(1).
Meteorite impacts may leave indentations but also cause the fact that lunar seas or the flat dark areas on the Moon occurred because of hot magma rising through fractures in craters, from which the lava originated.
(1) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "What are the large dark areas on the Moon?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 97. Print.
(2) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "Why is the Moon covered with craters?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 96-97. Print.
Figure - 25.1 By Peter FreimanCmgleeBackground photograph by Gregory H. Revera - Remake of File:FullMoon2010.jpgBitmap from File:FullMoon2010.jpgOwn work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14580532
Figure - 25.2 https://www.windows2universe.org/earth/images/maria_3x4.gif
Figure - 25.3 http://meteorites.wustl.edu/lunar/moon_meteorites_files/image010.gif
Figure - 25.4 By NASA/GSFC/JPL/Colorado School of Mines/MIT - http://www.nasa.gov/sites/default/files/14-236_0.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=35898125
Figure - 25.5 By NASA/Colorado School of Mines/MIT/JPL/GSFC - http://www.nasa.gov/sites/default/files/grail20141001.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=35898240