"Nordeste" stretches from the Atlantic seaboard in the northeast and southeast, northwest and west to the Amazon Basin and south through the Espinhaço highlands in southern Bahia. It encloses the São Francisco River and drainage basin, which were instrumental in the exploration, settlement and economic development of the region. The region lies entirely within the earth's tropical zone and encompasses Caatinga, Atlantic Forest and part of the Cerrado ecoregions. The climate is hot and semi-arid, varying from xeric in Caatinga, to mesic in Cerrado and hydric in the Atlantic Forest.