I get your point, but that's not exactly the case. Madrid is built there not by coincidence, there are a lot of underground rivers that cross the region, but then the city was built on top of them.

See here: https://www-canaldeisabelsegunda-es.translate.goog/-/madrid-...

Original: https://www.canaldeisabelsegunda.es/-/madrid-la-ciudad-edifi...

And about ruins, they are, but they are also hidden: https://es.wikipedia.org/wiki/Muralla_cristiana_de_Madrid

:)