Mare Long ago, about the time that Galileo first pointed his telescope towards the Moon in 1610, some astronomers thought that the dark areas on the Moon were seas filled with water. The seas on the Moon are in fact relatively smooth areas caused by lava flows. This lava covered large regions of the Moon billions of years ago, covering and filling in many craters. The lava does not reflect light well and so appears dark to an observer on Earth. Galileo first discovered these features when he pointed his telescope to the Moon.
Craters are circular depressions caused by the impact of meteorites.
Some craters on the Moon were caused by volcanoes, however, lunar craters caused by meteorites far outnumber those caused by volcanic activity. Craters on the Moon can vary in size from less than one metre in diameter up to 1,000 km in diameter. Random House, 1991. R.L. Bishop, ed. Observer's Handbook 1998 .