Many old cities around the world have even older, past versions of themselves underneath. Even in the U.S, where cities are barely a few hundred years old, there are vast subterranean ruins of older cities.

When I was in Scotland, I went on a "haunted" tour of the entire underground medieval Edinborough. I was impressed that the former "street" level was 20 or 30 feet lower than the modern day streets. How does this happen?

When I was in Barcelona, 20 or 30 feet below the city was an entire Roman colony!

Why do cities seem to have this tendency to creep upwards? How do they do it? Is there a way to observe it in modern cities happening in real time?