The columns are made of a hard rock called basalt. They formed about 60 million years ago, when red-hot melted rock from inside the Earth came up through cracks in the ground. As the rock cooled down slowly, it shrank and split into these neat shapes - a bit like the way mud cracks into shapes when a puddle dries.
Most of the columns are hexagons (six-sided). But there are also pentagons (five), squares, sevens and eights. There are around 40,000 of them altogether, stretching down into the sea.
There is a wonderful old folk tale about how the Causeway was made. The story says a giant called Fionn mac Cumhaill (FINN mac-COOL) built it as a stepping-stone path across the sea so that he could go and fight another giant. Of course the real reason is the cooling lava - but the story is much more fun.
The Giant's Causeway is a UNESCO World Heritage Site, which means it is protected as important for the whole world. Around a million people visit every year. The columns are smooth and slippy where the sea has worn them down, so most visitors hop carefully from stone to stone.