Imagine what they would find if they looked on the ocean floor (down to about 200-300' deep). Sea level has previously been much lower (and sometimes a bit higher) than it currently is. Given the current long term (Milankovitch) cycles, Earth spends most of its time in a much colder state, with significantly lower sea levels with only relatively brief periods of higher sea levels. Around 30-40 thousand years ago, sea levels were about 100m lower than they currently are, with the current high sea levels only occurring in the last 20k years.
Most human civilizations congregate around the coasts of seas for access to transport, favorable weather for growing crops (less extremes due to the massive heat capacity of oceans) and sea food. Traces of civilizations that existed back then will likely be hard to find, but no main stream archeologist will ever look, because finding them would shatter their "consensus" view.
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