Well, as you can see, it was pretty impressive. Basically, the "crown jewel" of the Causeway (which you can kind of see in the top picture) is the pathway of hexagonal (sometimes pentagonal) basalt pillars that leads out into the ocean. The same pillars can also be found at other places around the Causeway, but the picture above shows part of the main formation.
While geological and meteorological history are certainly interesting, one of the factors that makes the Giant's Causeway so fascinating and magical comes from the realm of myth and legend. Which, I have to admit, is much more to my taste. According to ancient Irish legends, the causeway was built by the giant Finn McCool (Irish: Fionn mac Cumhaill). Apparently, you see, there was this running feud between Finn and a Scottish giant, Benandonner, who, obviously enough, lived across the Irish Sea in Scotland. Well, every day, Finn would stand up on a mountain near the future sight of the causeway, and he and the Scottish giant would shout obscenities at one another across the ocean. One day, however, something that one (or both) of them said - an insult against one of their giant mothers, perhaps? - pissed both of them off so much, that they decided they'd build a bridge across the sea and engage in some good, old-fashioned fisticuffs.
So off they went, building the bridge from their respective homelands. Well, halfway through his work on his side of the bridge, Finn McCool looked up and saw Benandonner off in the distance. Even from that distance away, he realized that the Scottish giant was substantially bigger than he was. Which, of course, didn't bode well for the upcoming fight. So he did what any of us would do in such a situation: he ran off home to his wife and begged her to help him. Being the brains of the McCool clan, Mrs. McCool came up with an ingenious plan. She dressed Finn up as a baby and put him into a cradle.
Well, pretty soon, Benandonner had finished the Scottish-Irish landbridge and had arrived in Antrim looking for Finn. Upon coming to his adversary's house, Benandonner (politely?) asked Mrs. McCool where the deuce her husband was. Mrs. M told the Scottish giant that her husband was off in the countryside for a bit, but that he would be returning home shortly. Meanwhile, she invited Benandonner inside to wait for Finn to "return." Upon entering the house, however, Benandonner noticed the "baby" in the enormous cradle and stopped dead cold. Giants have never been known for their cognitive capabilities (see above), and the gas in Benandonner's mental tank seems to have been running on empty. Regardless, he took one look at the so-called baby and thought to himself: "Shite! If this is the size of Finn McCool's baby, then Finn himself most be one big, bad son-of-a-leprechaun!" Promptly excusing himself from Mrs. McCool's hospitality, Benandonner fled back across the Irish Sea, tearing up the causeway as he went.
And all that's left of that original giant's causeway, is what we can see in the present day at Giant's Causeway. And now you know.