As to the origin of the lunar craters, some believe that they were produced in past ages by a bombardment of the lunar surface by huge meteoric masses; but there are many objections to this theory that we will not take up here. It is more generally believed that the lunar craters are a result of volcanic activity on an enormous scale which took place on the moon many ages ago and which has now practically ceased, its only manifestations now taking the form of a quiet emission of gases from small volcanic vents or fumaroles which exist all over the lunar surface but which are to be found in greatest numbers on the floors and sides of craters.