Many attempts have been made to compare the smaller features of the moon with earthly features. Thus such familiar structural items as lava flows, faults, domes, and pressure ridges have been delineated. The larger lunar features—the maria, the mountains, the clefts, and the larger craters—have not heretofore been thought to have earthly counterparts. There are two principal theories of origin of the lunar features—volcanic and meteoritic. Both theories lead to the conclusion that the agent which produced the structures was more active in the past and may have been essentially dormant since the Archeozoic era. The most likely place on earth to search for similar features is in areas that have remained little changed since that time.

In many ways the floor of the Atlantic Ocean resembles the moon's surface. The Madeira Abyssal Plain is similar in size and shape to Mare Crisium; the Sohm Abyssal Plain is very like Oceanus Procellarum or Mare Frigoris; the Mid-ocean ridge is reminiscent of the lunar uplands; mid-ocean canyons recall the clefts of the moon; and the sea-mounts are like the isolated peaks and mountains of the Mare Imbrium district. East of Bermuda is a range of low hills (abyssal hills) arranged in a fan-shape very similar to the lunar Haemus Mountains. Two gashes across the Mid-Atlantic ridge, one at Lat. 30°N. and one at the equator, are very like the Alpine Valley of the moon. Steep slopes are characteristic of both the lunar mountains and the submarine slopes of the Atlantic. Both the Atlantic floor and the topographically low areas of the moon are thought to be blanketed with dust-sized material.

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