Archaeology is the scientific study of past human societies through excavation. Archaeologists attempt to discover and describe past cultures and explain their development. An archaeological site is chosen based on artifacts found on the surface or the historical context of the area. A unit is a square area that archaeologists excavate to search for artifacts. Tools used include shovels, trowels, measuring tapes, and brushes. Careful excavation by removing soil in 5cm levels allows artifacts to be found and mapped for analysis. The process continues until sterile soil is reached.