Columnar jointing forms much the way drying mud cracks. Cooling lava contracts, and as the chill works inward from the surface the shrinking forces find equilibrium and split. Those splits propagate downward into columns. Hexagons are theoretically the most stable, so hexagonal columns dominate, with fives and sevens mixed in. At Daepo-dong the columns stack into a cliff over twenty metres high, and the sheer scale is part of why it looks manufactured.
Jeju is a volcanic island, so this kind of formation appears in many places, but Daepo-dong is treated as the representative site because the columns are so clearly defined and stand directly against the sea. When waves hit the base, water is forced up through the narrow gaps between columns and shoots skyward; on rough days those spouts can reach the height of the cliff. Just after a typhoon, or on a windy day, your chances of seeing it are good.
Viewing is from a boardwalk along the clifftop looking down. There is no path to the bottom; several viewpoints simply offer different angles. Staying behind the railing matters here — the drop is high and the sea is directly below. The walk is short, thirty minutes is enough, and the path is built to take strollers and wheelchairs.
Things to see cluster nearby. Jungmun Saekdal Beach is within walking distance, and Cheonjeyeon Falls and the Yeomiji Botanical Garden are close. On a southwestern Jeju itinerary it is normal to treat this area as a half-day loop. Since the cliff itself takes only half an hour, it works better folded into nearby plans than as a destination you travel far to reach.