The Giant's Causeway in Northern Ireland is a natural wonder made up of around 40,000 interlocking basalt columns that look like stepping stones leading into the sea. Legend has it that these columns were built by a giant named Fionn mac Cumhaill to challenge a Scottish giant. The site became popular with tourists in the 19th century and now attracts over 998,000 visitors annually. Managed by the National Trust, the Giant's Causeway offers free access to its unique geological formations and stunning coastal views.