South America is a geographically challenging land mass. The bulk of its territory is located in the equatorial zone, making nearly all of the northern two-thirds of its territory tropical. Jungle territory is the most difficult sort of biome to adapt for human economic activity. Clearing the land alone carries onerous costs. Soils are poor. Diseases run rampant. The climate is often too humid to allow grains to ripen. Even where rivers are navigable, often their banks are too muddy for construction, as with the Amazon. As the tropics dominate South America, the continent's economic and political history has been problematic. Venezuela, Guyana, Suriname and French Guiana are fully within the tropical zone, and as such always have faced difficulties in achieving economic and political stability, though the discovery of oil in Venezuela improved that country's economic trajectory.