It’s sort of like saying there’s an underground passage system in Walt Disney World’s Magic Kingdom. It’s actually not – it’s the 1st floor and Main Street is technically the 2nd floor with the ground built up around it. That’s true with most cities or monuments we’ve uncovered over the years, it’s that stuff has grown/blown around things and hidden the items underneath. So for example, the Sphinx was completely covered with sand when it was first found – couldn’t see it at all the way we do today simply because of sand storms that blew the sand around it.
For found “cities”, its overgrowth of the surrounding land, trees, vines, bushes, whatever that continues to go thru a cycle of regrowing and dying, causing matter to build up underneath that mulches up and becomes dirt. Over the years (centuries or more) – it eventually gets buried from this cycle along with any other natural events that occur such as trees growing and falling, flooding, etc. that can easily cover things up.
In Pompei, it was a volcanic eruption that entombed the city with feet of ash and soot from the initial blast (pyroclastic cloud) of debris that was superheated that killed people resulting in the homes and buildings being buried but also being being killed instantly.
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