Throughout the history of the oceans, which goes back about 3.5 billion years, give or take a few million, climate has constantly changed and, in response, sea level has gone up and down.
People have been keeping track of sea level, or the elevation of the oceans, for about 200 years. Until fairly recently, this was done with tide gauges, which are water-level recorders anchored to some structure along the coastline. It might be a wharf, a concrete breakwater or some other solid structure that is stable over long periods of time.
A tide gauge is essentially a large pipe inserted into the ocean, which has a float inside that moves up and down as the water level changes. As the tide rises and falls each day, these gauges record those changes in water level, day after day, year after year.
These instruments were first set up to provide accurate information on water depths so ships could enter and leave ports safely.
In order to bring some order to all of these geographical variations, and to provide a constant point of reference, a datum or base level was established based on averaging out the elevation of sea level from many tide gauges over an extended period of time.
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