How do acient cities end up underground? Like the Palatine under Rome.?

Answer 1

There are ones buried under ash from a volcano like Pompeii was by Vesuvius... That's more immediate.
Other places must have been abandoned due to raids and conquering or disease and then the elements got to the cities... like soil erosion.. dust and dirt blown in by the wind over decades or centuries.
When people go back to an abandoned area that might be overgrown and hidden under dirt and or ash they build building on top of ruins that were forgotten about. The stones seen on top were probably used as foundations for new buildings... Later, when people got 'smarter' on how to built stronger structures they were using machines to dig and 'found' the old ruins.

It always amazed me how big cities seemed to get 'forgotten' in history or 'lost'... it's huge! How many people lived there?
Then centuries later there are clues to indicate where these great cities are.... I 'dunno' seems weird to me.

Basically, it doesn't happen overnight. There were far fewer people back then so relocating a town due to disaster may not have been that big of a deal.

I visited an ancient Roman city in Turkey (Ephesus) that has been excavated... to my surprise the guide told us it used to be a bustling "harbour city"! We rode in the bus for well over 30 minutes to get there from the sea side!
Boy had the landscape changed in what 1000 years?! It was a great experience to visit it.

Answer 2

thousands of years of erosion, and development.