Excavation is a primary way in which archaeologists acquire subsurface data about the past. Today, excavation is the strategy of last resort, as it is potentially destructive. Ideally, multidisciplinary research teams made up of specialists from several disciplines work together with a carefully formulated research design. Sites can be excavated totally or, as is more common, selectively. Vertical excavation is used to test stratigraphy and to make deep probes of archaeological deposits. Horizontal or area excavation is used to uncover larger areas and especially to excavate site layouts and buildings. The process of archaeological excavation begins with a precise site survey and establishment of a site recording grid. Placement of trenches is determined by locating likely areas or by sampling methods. Careful stratigraphic observation in three dimensions is the basis of all good excavation and is used to demonstrate relationships among layers, as well as between layers and artifacts. Excavation is followed by analysis and interpretation, and finally, publication of the finds to provide a permanent record of the work carried out. Among the special excavation problems are the recovery of fragile objects, human skeletons, and structures and pits.