How come archaeological ruins are always underground?
How come archaeological ruins are always underground? Think about it. Why isn't everything right on the surface? Where does this dirt come from that keeps burying the past? Is the Earth getting thicker and thicker, like the trunk of a tree? Doesn't make sense to me.
Actually, Nig — and listen, you really should do something about that nickname — I like to think the earth is getting slightly less thick each year, owing to my selfless educational ministry. Physically, on the other hand, the earth is getting a bit thicker, since it picks up 10,000 tons of meteorite dust a year. But that's not why ruins are buried.
Archaeologists have to dig for lots of little reasons and one big reason. Sometimes the stuff they're looking for was buried to start with, as in the case of graves and rubbish pits. Sites that are abandoned for a long time become overgrown with vegetation that gradually decays and builds up a layer of topsoil. Places located in valleys may get covered by erosion from nearby hillsides. Occasionally a site gets buried because of some natural disaster, such as a flood or the eruption that buried Pompeii. The great Egyptian temple at Abu Simbel (the one with the giant seated figures carved into a cliff) was partly buried by drifting desert sand. The same thing happened to the Sphinx — for centuries all that was visible was the head. The Roman port of Ostia was also engulfed in sand, which accounts for the remarkable state of preservation in which modern excavators found it.
The major reason archaeologists have to dig, however, has to do with the peculiarities of human settlement. Towns don't get built just anywhere; they're usually located near water, transportation routes, fertile land, etc. A good location may be deserted once in a while due to war or disease, but generally it's soon reoccupied. In the ancient world many places were continuously inhabited for thousands of years, being finally abandoned only after some change in external circumstances — say, deteriorating farming conditions or one malaria outbreak too many.
Then we get to the matter of (ahem) shoddy home construction. You may think this problem only dates back to the invention of aluminum siding, but not so. In many parts of the world, Mesopotamia (modern Iraq) for one, the principal building materials were mud or mud brick, neither of which was very durable. When a mud house collapsed, as it inevitably did sooner or later, the owner went off to find more hospitable quarters and rain reduced what was left to a flat pile of mush. Eventually some mope scrounged up more mud and built a new house on top of the old one.
Meanwhile, trash and sewage were piling up in the streets. After a few centuries of this the prevailing grade rose to such an extent that the town wound up sitting on an artificial hill or mound. Wholesale destruction due to war or fire obviously accelerated things.
If and when the site was finally abandoned, natural forces gradually reduced it to an odd bump on the landscape. It might even be farmed, since it was basically just a big mud pile. Archaeologists have learned to look for these mounds (called tells in the Middle East), which have concealed what's left of places like Troy, Babylon, and the biblical city of Nineveh. They have to dig especially deep to find things like temples, because these generally were kept free of trash and in good repair, meaning that their grade did not rise with the surrounding city. Many temples, in fact, were semi-buried even in ancient times.
Cities built of more durable materials like stone or fired brick are usually not completely buried. The monuments of Rome, for example, have always been visible, even though prior to the start of serious archaeological work some were half-buried due to siltation, plant overgrowth, trash accumulation, and so on. The real problem was medieval and Renaissance contractors carting away parts of old buildings to use in putting up their own. (That's what happened to most of the Colosseum.)
In some cases, not just in Rome, buildings were completely razed and new structures built on the old foundations, providing yet another lode of archaeological ore — a fact that must give us pause, given the state of many modern basements. God knows what future archaeologists are going to make of the five million old egg cartons my mother-in-law's got. Clean up that mess today, lest you make us look bad in the eyes of scientists yet unborn.