The excavation that forms the grave. Excavations vary from a shallow scraping to removal of topsoil to a depth of 6 feet (1.8 metres) or more where a vault or burial chamber is to be constructed. However, most modern graves in the United States are only 4 feet deep as the casket is placed into a concrete box (see burial vault) to prevent a sinkhole, to ensure the grave is strong enough to be driven over, and to prevent floating in the instance of a flood.