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Archaeologists first look for clues, such as bricks, stones or bits of broken pottery ('sherds'). They study photographs taken from planes or balloons, because aerial photos may show ruins hidden beneath the ground.
Before excavating a site, a team of archaeologists marks out a grid, in squares. They make an excavation map of the site. They work square by square, clearing away soil. They remove finds (artefacts, bones, plant seeds) very carefully. They mark each find, and where it was found. They work carefully, because most very old things are easily broken.
They may use a scanner to look underground. A geophysical scan can show up a buried building.
What is How do archaeologists explore a lost city?