Cairns are used as trail markers in many parts of the world, in uplands, on moorland, on mountaintops, near waterways and on sea cliffs, and also in barren desert and tundra areas. They vary in size from small stone markers to entire artificial hills, and in complexity from loose, conical rock piles to delicately balanced sculptures and elaborate feats of megalithic engineering. Cairns may be painted or otherwise decorated, whether for increased visibility or for religious reasons. An ancient example is the inuksuk (plural inuksuit), used by the Inuit, Inupiat, Kalaallit, Yupik, and other peoples of the Arctic region of North America. These structures are found from Alaska to Greenland. This region, above the Arctic Circle, is dominated by the tundra biome and has areas with few natural landmarks.
In modern times, cairns are often erected as landmarks, a use they have had since ancient times, but since prehistory they have also been built for a variety of other reasons, such as burial monuments, and for defence and hunting, as well as ceremonial, astronomical, and other purposes.