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. –Nig Lipscomb, Chicago

As a matter of fact, Nig–and listen, you really should do something about that nickname–indications are the earth is getting slightly less thick each year. I attribute this largely to my saintly educational influence. 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 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, the principal building materials were mud or mud brick, neither of which is very durable. When a mud house collapses, as it inevitably does sooner or later, the owner goes off to find more hospitable quarters and rain reduces what’s left to a flat pile of mush. Eventually some mope scrounges up more mud and builds a new house on top of the old one. After a few hundred iterations of this process the prevailing grade rises to such an extent that the town winds up sitting on an artificial hill or mound. Wholesale destruction due to war or fire obviously accelerates things.

If and when the site is finally abandoned, natural forces gradually reduce it to an odd bump on the landscape. It may even be farmed, since it’s basically just a big mud pile. Archaeologists have learned to look for these mounds, which have concealed what’s left of places like Troy, Babylon, and the biblical city of Nineveh.

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–a fact that must give us pause, considering the sorry state of most modern basements. God knows what future archaeologists are going to make of the five million old egg cartons my mother-in-law’s got. Do your bit for posterity and get that mess cleaned up today.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.