Archaeology is the scientific study of past human societies through excavating physical evidence. An archaeologist's site is chosen based on artifacts or history found on the surface or from sources. A unit is a square area an archaeologist chooses to dig within. Setting up a 2x2 unit involves measuring sides that are 200 cm and calculating the hypotenuse. Tools used include shovels, trowels, measuring tapes, and brushes. First the plow zone to 25 cm below the surface is removed, keeping an eye out for artifacts. Below this, changes in soil color indicate features that may contain more artifacts. Careful excavation of 5 cm levels allows examination of each soil layer.