Apparently the streets of the Old City of Jerusalem from the time of Jesus are now about 12 feet underground. There are excavated walls in Jerusalem from about 1000BC which are even deeper than that, and I’ve heard the same thing about the streets of London. Why are they so far underground? I get that you might have a bit of debris pile up, and that people might build new roads on top of older roads and so on, but 12-15 feet? In cities that have been consistently inhabited throughout that time? How does that happen?
Cecil’s tackled this one, long ago, with a focus on archaeology: http://www.straightdope.com/columns/read/813/how-come-archaeological-ruins-are-always-underground
One word: Tell.
Of course, if an ancient city is built somewhere that’s eroded away, it’s lost and archaeologists can never find it by digging. So ancient cities might be still at the surface (and still obvious), buried (and thus discovered by digging) or simply eroded (and lost forever).
The links provided explain why cities tend to deposit layers of material over time, and people don’t settle in areas of high erosion if they can help it, but through all of history there must be more than a few cities that have eroded away.
Often you get deliberate fill.
For example, one of the archeological sites that my mother’s cousin worked dug at has a layer known as the Roman fill layer. In this case, rather than removing all of the fallen debris after an earthquake, the then owners of the area filled the site with several feet of dirt and garbage (broken pottery, etc) and built on top of that.
It’s not that the older cities sink, it’s that the newer ones rise on top of them.
Not sure that will be true of cities in the future, though.
One way to think of it - they don’t all end up underground, plenty end up at ground level or floating up in the air. But these latter two destinations also mean the ancient things get erased or otherwise disappear.
This doesn’t apply to all cities, but many ancient cities were founded next to rivers and other water sources. Water rolls downhill, and thus the oldest cities are build at the bottom of a hill, in a depression, or in a large valley. That makes it easier for erosion, earthquakes, and fires to bury it over time.
An exercise: leave some stuff outside for a year or two. Don’t move it or clean it. Continue to use the area as before. Depending on where you live, it is probably covered in leaves, dust and crud by now.
Imagine that process for 2000 years.