While every Korean child is taught about this since elementary school, the Dolmen Sites of South Korea are unfortunately, often overlooked.
A dolmen site is an ancient form of tomb. A famous example would be Stonehenge in the United Kingdom. South Korea has the largest concentration of such sites in the world, accounting for about 40% of the planet's dolmen tombs.
The sites are mostly concentrated in three areas: Ganghwa (near Incheon), Gochang (pictured, north of Gwangju), and Hwasun (south east of Gwangju). For their significance to humanity, the three sites have been granted World Heritage status by UNESCO.
I have been to Ganghwa and Gochang. My friend who has been to Gochang and Hwasun says the latter is the best. Ganghwa has limited public transport and all dolmens are pretty scattered. Only go if you have a car, or an incredible amount of patience. The Gochang site is a short bus ride away Gochang bus depot. All tombs are next to each other and adjacent to the Gochang Bio Reservior, a potential World Heritage Site in itself. Hwasun is the closest to a big city and said to be the easiest to access.