Not all sites in the Superfund Enterprise Management System (SEMS) need to be cleaned up. Once a site is listed in SEMS, EPA conducts a site investigation and assessment of the contamination. If these studies show that the site is safe, the site is deleted from SEMS and no further cleanup action is taken. These sites are called "archived" sites. Sites that pose a significant threat to human health and the environment are listed on the National Priorities List (NPL) and cleaned up by potentially responsible parties (PRPs), EPA, the state or tribe. Sites that contain hazardous waste below levels that capture EPA's attention are referred to the states for cleanup.