The USGS archive contains Level-1 data collected by the Landsat satellites from 1972 to present.
Data is downlinked to various ground stations around the world - and while each ground station in the International Cooperator (IC) network is the primary source for distributing the captured data in their area, copies of the data have also been made available to the USGS to place in the Landsat archive for distribution via EarthExplorer, GloVis, and the LandsatLook Viewer.
Historial data not found in the USGS archive may be available from the ground station that collected the data. The Landsat International Ground Station Network web page displays contact information and coverage maps.
In 2010, the Landsat Global Archive Consolidation (LGAC) effort began, to consolidate the Landsat data archives of all international ground stations, make the data more accessible to the global Landsat community, and significantly increase the frequency of observations over a given area of interest to improve scientific uses such as change detection and analysis.