Worldbuilding - Explore Every Impossibility

For my world, I am trying to create abandoned cities that archeologists would later discover. However, I am trying to justify why they were abandoned and forgotten in the first place.

But first, let me remind you there are two types of abandoned cities. The first type are the weird, mysterious ones, whose architecture is inspired by Alien and the art of H. R. Giger. They need no explanation, for they are supposed to be mysterious and eerie. The second type are the regular abandoned and lost cities. They are more recent (in comparison to the weird cities), and these are the ones thst were founded by ancient people and races.

For cities over time, how could I explain for them being completely abandoned?

Note: By abandoned, everyone has left and it gets covered over time, cities that were built-over do not count.