What caused them to go “underground”?
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The weather and elements remove material from higher places (mountains, hills) and deposit them in lower places (e.g. river valleys). Over time the material (sediments) accumulate and cover whatever was there before them.
In case of ancient cities there is also another process – humans tend to build new stuff over older foundations and whatever rubbish that has accumulated under their feet. So over centuries cities rise up, with all the old stuff piling up underneath. So many ancient cities can now be seen as mounds which archeologists can dig in.
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