From the Semitic root for "ruin", the name given in archaeology to an artificial mound built of successive layers of mudbrick and refuse left by continuous levels of occupation. The mounds can reach impressive heights; Megiddo in Palestine stands at over 70 ft, while Lachish stands at over 100.
The general Near Eastern landscape and climate is rather hostile to wide-spread habitation without human intervention. The rainfall is on average 200-250 mm annually, well below the 300-350 mm required every 3 out of 5 years to support subsistence agriculture. The solution, at least since the Ubaid period, has been to build settlements on the occasional strips of land irrigated naturally by rivers, and to develop canals to extend the water supply into the surrounding fields. A natural side-effect of this process is that particular sites are continually inhabited over long periods of time; there are few sites suitable for development, and it is easier to maintain earlier canals and settlements than to start afresh.
As the settlements expand, basic buildings made of mudbrick and reed deteriorate, while even sturdier buildings of stone are destroyed and rebuilt. As each area is destroyed, the materials are leveled and used as foundations for their replacements. Occasionally, an entire city is destroyed in war or burned by a major fire, and these too become layers in the archaeological record. Even when the city is finally abandoned and becomes covered in sand and soil, it is often still used today as pasturage for a local village or nomadic group. The Tell El Jib, the site in the earlier part of this century of a commonly used well, was found after digging to be the site of the ancient city of Gibeon. The layers thus form a datable series of strata, forming an archaeological record of the political changes in a city.
Countless tells dot the landscape of the Near East, most of which remain unexcavated. Usually a site is chosen after a careful surface survey, collecting potsherds which have settled to the top of the mound. If the surface survey is favourable, the next step is usually to dig a step trench, basically cutting an insection into the mound from which a representative portion of the layers are visible; recent excavations at Tell Hamoukkar in Syria have revealed layers of walls from the Islamic, Persian, and Assyrian periods, all the way back to the Sumerian in the bottom-most layer.