Human geography examines how people organize space, from sprawling megacities to global networks of trade and resources. A megacity is a city with more than 10 million inhabitants; examples include Tokyo (the largest metropolitan area, with about 37 million people), Delhi, Shanghai, São Paulo, Mexico City, Cairo, and Mumbai. Urbanization, the shift of population from rural to urban areas, has accelerated dramatically over the past century. India is the most populous country as of 2024, surpassing China, which together with Russia each borders 14 other countries, the most of any nation. Russia remains the largest country by area at roughly 17.1 million km², while the United States is the world's largest oil producer.
Energy, water, and infrastructure connect geography to economics. OPEC, the Organization of the Petroleum Exporting Countries, is a cartel of oil-producing nations that coordinate production and pricing. The Three Gorges Dam on China's Yangtze River is the world's largest hydroelectric facility by capacity, illustrating how massive engineering reshapes rivers and landscapes. The Trans-Canada Highway and Australia's Highway 1 are among the longest national highways in the world. Brazil holds the largest renewable freshwater resources on the planet, a reminder that water is distributed very unevenly across the globe.
Geography is also a story of basins, seas, and oceans. A watershed, or drainage basin, is the area of land where all surface water drains to a common outlet, such as the Mississippi River, which drains into the Gulf of Mexico, or the Volga River, which drains into the Caspian Sea. An endorheic basin is a drainage basin with no outflow to the ocean, like the Caspian basin itself. Oceans are larger and continent-bound, while seas are smaller and often partly enclosed by land, a distinction that helps explain why the Mediterranean, the Red Sea, the Persian Gulf, and the Aegean are seas rather than oceans. Together, these physical features and human patterns form the layers of the world that maps, GIS, and GPS now help us measure, navigate, and understand.