When thick basaltic lava cools slowly enough, it contracts. The contraction has to go somewhere, and it goes into a network of fractures that propagate from the cooling surface downward. Where the rate of cooling is steady and the lava is right, those fractures meet at angles that distribute stress most efficiently - and the most efficient angle, for a flat sheet contracting from above, turns out to be 120 degrees. Hexagons tile. The Giant's Causeway is what happens when a 28-metre lava flow cools through six-sided geometry on a scale a person can walk on. The Palaeocene rocks here were chosen in October 2022 by the International Union of Geological Sciences as one of the first 100 geological heritage sites in the world - not just for what they look like, but for their role in establishing volcanology as a science. UNESCO had already declared the Causeway and Causeway Coast a World Heritage Site in 1986; the Department of the Environment declared it a national nature reserve a year later.

The other story is harder to date and easier to remember. The Irish giant Fionn mac Cumhaill - Finn MacCool in English - was challenged to a fight by the Scottish giant Benandonner, and Fionn built a causeway across the sea so the two could meet. In one telling, Fionn wins. In a better telling, Fionn climbs back to Ulster, looks across the water at Benandonner, and discovers that his opponent is much, much bigger than he had assumed. His wife Sadhbh dresses him as a baby and tucks him in a cradle. When Benandonner arrives and sees the size of the infant, he flees back to Scotland, breaking the causeway behind him so Fionn cannot follow. Across the North Channel, the same lava flow surfaces again at Fingal's Cave on the Scottish isle of Staffa - identical basalt columns. The geology and the legend agree on one thing: there is a connection between these two places. The legend is just funnier about it.

William King, Bishop of Derry, came to look at the rocks in 1692. A paper by Sir Richard Bulkeley of Trinity College, Dublin, took the news to the Royal Society the following year. The Causeway became known to the wider European world in 1739, when the Dublin artist Susanna Drury made watercolour paintings of it. Drury won the first ever award of the Royal Dublin Society in 1740 for those paintings. The watercolours were engraved in 1743, and the engravings travelled. In 1765 they appeared in Volume 12 of the French Encyclopédie, and in 1768 the engraving of the East Prospect appeared in a volume of plates. In the caption to that plate, the French geologist Nicolas Desmarest suggested, for the first time in print, that columnar structures like the Causeway were volcanic in origin. The Causeway, in that sense, helped invent volcanology - not by being studied first, but by being looked at hard enough.

Visitors over the centuries have given names to weathered shapes in the basalt. The Organ. The Giant's Boot. The Chimney Stacks. The Giant's Harp. The Camel's Hump. Reddish low columns are called Giant's Eyes; uneven stacks are the Honeycomb. Fulmar, petrel, cormorant, shag, redshank, guillemot, and razorbill nest along the cliffs. Sea spleenwort, hare's-foot trefoil, vernal squill, and frog orchid grow in the rock. In October 2011, a colony of stromatolites - the layered microbial structures usually associated with warm, salty water - was reported here, an unexpected find for this latitude. The National Trust manages most of the site. Access to the stones themselves is free. The visitor centre charges a fee but is not the only way in, and the half-mile walk down from the entrance ends with people quietly stepping from column to column as if they had been invited.

Emperor Pedro II of Brazil came to the Causeway on 9 July 1877, in the middle of an unpublicised three-day visit to Ireland. The site was already a tourist destination then. It got busier when the Causeway Tramway opened in 1883, hauling people from Portrush through Bushmills to within walking distance of the stones. The National Trust took over in the 1960s and removed the worst of the commercial clutter. The first permanent visitor centre opened in 1986; it burned down in 2000; the present one opened in 2012, funded by the National Trust, the Northern Ireland Tourist Board, the Heritage Lottery Fund, and public donations. In 2018, more than a million people walked through it. The basalt columns have been at the same address for sixty million years and counting. The tour buses come and go.