Borders define geographic boundaries of political entities such as governments, sovereign states, and other subnational entities.
A country’s borders continue beneath the surface all the way to the earth’s core.
Above the ground, boundaries extend 100 miles (161 km) into the atmosphere.
That means that airlines must get permission to enter the air space of each country they fly over.
Natural borders are geographical features that present natural obstacles to communication and transport.
Existing political borders are usually a formalization of these historical, natural obstacles.