The world's largest active archive of global weather data, the National Climatic Data Center provides weather data to researchers all over the world. The NCDC gathers and maintains weather data from some 10,000 weather stations around the United States, and some of its historical data goes back more than 200 years. From 2004 to 2013, NCDC’s digital archive increased from 2 to 14 petabytes. A petabyte is 1,000,000,000,000,000 or 10 to the 15th power bytes. Users of the data range from large engineering firms planning energy-efficient development to individuals planning a retirement move. At present, only group tours of the center are available and must be arranged in advance. Although the NCDC is an incredible source of climate and weather data, there isn't much to see for individual visitors.