Over hundreds and hundred of years this can create a layer effect. As scientists dig down through the mound, they see various layers for each rebuilding of the city. If a city is around for a thousand or more years, a dozen layers may have built up, with each layer representing a different period from the history of the city. Sometimes even when the remains of an old city can't be seen from the ground, it can be seen clearly from the air. Slight depressions in the soil over an old ditch or a bank over an old wall maybe seen from an airplane in the early morning or evening when shadows are long.
Crop marks can also give clues to buried features.
Plants will grow shorter and more yellow over buried walls and taller and greener over old ditches and pits. As the world has gone more high-tech, so have the methods scientists use to find old buried ruins. Infrared photographs taken from a plane can detect temperature differences on the surface of the ground due to buried features. It is possible also to detect underground features by using probes to measure the electrical resistance of the soil or changes in the Earth's magnetic field. Sometimes finding a site depends not on observations made at the site at all, but by carefully studying old records and documents, or tracing down legends.