A cairn is a human-made pile of stones, usually built as a trail marker or memorial. Hikers often encounter cairns on mountain paths, where someone has carefully stacked flat rocks on top of each other to mark the correct route. In places where trees are scarce or fog rolls in quickly, these simple stone towers help travelers find their way.

People have built cairns for thousands of years. Ancient cultures created them to mark burial sites or important locations. Arctic peoples used inuksuit (a special type of cairn shaped like a person) to guide travelers across frozen landscapes. Scottish and Irish settlers built cairns on hilltops to honor the dead or commemorate significant events.

Today, you'll find cairns on hiking trails worldwide. When the path becomes unclear across a rocky slope or alpine meadow, a cairn might be the only sign pointing the right direction. Some hikers add stones to existing cairns as they pass, though park rangers often ask visitors not to build new ones, since too many cairns can confuse rather than help.

If you've ever carefully balanced rocks on a beach or stream, you've built a miniature cairn.