The sea surface is highly variable insomuch that the current sea level is far from being an average level in relation to the Earth’s surface and to the geological history of Earth. This means that the total surface area cannot be precisely defined, due both to its continuous change in altitude but also to the complexity of the landward limit, or the coastline. Sea level variations have been very intense in geological history, also in its recent Quaternary history, with a variability of more than a hundred meters. The surface of the sea clearly distinguishes two separate worlds, the emerged one and the submerged one, the former being easily accessible, the latter much less. Geological theories have been deeply affected by this border because until one century ago, little information was available on the sea bottom, so geologists could not produce really data-based global theories. It seems that geological theories were altogether land-laden, not being able to acquire information on the seabed. Even today the submerged lands continue to be much less accessible and certainly not directly available to the classic geological field surveys, apart from limited shallow areas where it is possible to use scuba equipment.
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