As opposed to man-made structures, natural structures are those forms that stand on their own and take a specific configuration but are not made by humans, including things like beehives, rock arches, canyons, eggshells, bird's nests, coral reefs and naturally formed caves. One way to understand natural structures is to look at the difference between natural structural phenomena such as mountain ranges compared to man-made structures such as houses and highways. Natural structures are created from the Earth's natural forces or through the efforts of animals.
The definition of a structure typically refers to something that is built or created. While humans can build things using tools, natural forces are also responsible for building things. The Grand Canyon, for example, was built from years and years of rock sediment accumulation followed by a river carving out the canyon's shape.