I have heard of cities that have been 'built on top' of another city. Supposedly you could take tunnels down and find complete streets and buildings. Why would they do this as opposed to knocking down what was there and just rebuilding at the same level? Wouldn't there be serious risk of collapse?
Or is this simply urban legend and doesn't actually happen? I can't actually recall a city that this would apply too (maybe New York or Egypt?) but nothing solid.
People have mentioned tells, but I'd like to give a bit more detail.
First off, be aware that urbanization (the process of societies agglomerating into cities) started becoming significant in the 4th millennium BCE, so well over half of the entire history of city-building occurred before Christ. Places like Ur, Lagash, and Babylon have very long histories of habitation.
Secondly, for consideration of Mesopotamian cities, the manner of construction is relevant. Lacking accessible forests and major quarries, most buildings were made of mud-brick (baked occassionally, but more often sun-dried).
When you wanted to construct a new building on extant city property, it's much easier to just level the previous space and build on top of the rubble, with the material of the prior building serving as your new "ground" floor, rather than attempting to remove all the old material. This causes the height of the city to rise incrementally over time.
Over such vast periods of inhabitation, disposal of waste becomes a matter involving quite a lot of mass, and that's another factor contributing to the city's gradual increase in elevation. There were no landfills or garbage men, so as rubbish (broken pottery, animal bones, etc) accumulated in streets, the simplest method of waste-management was to merely "re-pave" by laying down a new layer of dirt to cover the rubbish. Over time, street-level rose considerably, and many excavations note that the interiors of buildings were lower than the exterior street, and their doorways were slowly made shorter and shorter by the growth of the land outside. This would eventually give another reason to rebuild at a higher-level on top of the older construction: you had to keep up with the street!
So, in summation, when you're building with dirt and clay over the course of thousands of years, there's no reason to not just treat the older layers of the city as the "ground" for any new construction.
This is how tells were formed, and it wasn't a minor process. Some of them are several stories above the surrounding plains!
This happened in Seattle; there was a fire in the 1880's and they just built over the original street level. Now "ground level" is where the second story used to be, but the original ground level is still there, and they have tours and stuff where you can go see it.
Or is this simply urban legend and doesn't actually happen? I can't actually recall a city that this would apply too (maybe New York or Egypt?) but nothing solid.
Here in California, Sacramento was hit by a series of devastating floods during the 19th Century. Some businesses were totally covered up by mud, and new buildings built on top of them. Some have been recently excavated, which you can tour if you're in the area. Most of the town was raised 10 feet or so, so there's still a bunch of businesses in Old Sac that you can tour that are below ground level.
This is the only example I can think of from modern times off the top of my head. In ancient times, you'd get fires and floods, and they'd come in and rebuild on top. Troy, for example, has a bunch of different layers which you can read about here.
you might like to see....
You may also find this story interesting, about re-exposing the medieval bridge in Rochdale, which had been covered up in the early 20th century.
http://www.bbc.co.uk/news/uk-england-manchester-19591150
From an archaeological perspective, it is often easier to knock something down and build over it than it is to completely remove older structures. This is particularly true of ancient mudbrick structures, whose constant cycle of decay and rebuilding led to the creation of massive Tells (or Hüyüks, depending on where you are) in the eastern Mediterranean and Near East.
This is what's known as a 'tel'. It is very common in parts of the world that have been inhabited for long periods of time such as the Middle East and the Eastern Mediterranean. Usually there is an original settlement and then some disaster (either natural like a flood or famine, or man-made like war) causes it to be abandoned and sediment builds up over time and later a new city is built on top of it. Often times it's not just one city built on top of another, but you'll have the modern city with half a dozen layers of cities below.
A good example of this is Tel Be'er Sheva, on the outskirts of modern day Be'er Sheva. You have the Iron age city with an original Copper Age city directly below it, and very close by you have the modern city built up since the Ottoman period with the Roman era city below it.
Do you mean Cairo when you say Egypt or just any city in general? If you mean Cairo, I don't believe what you described is exactly the case, most that if you excavate, you can find the remnants of previous cities including clear evidence of old streets and foundations for buildings.