The Giant's Causeway consists of about 40,000 interlocking basalt columns on the northeast cost of Northern Ireland, in County Antrim.

According to legend, the bad-tempered Irish giant Finn MacCool built a road across the waves to reach his enemy Finn Gall, who lived on Scotland's Isle of Staffa.

MacCool hammered a number of long, stone stakes into the seabed.

Before challenging Finn Gall to a duel, the Irish giant returned home to rest. Meanwhile, Finn Gall crossed to Ireland. When he saw the sleeping giant, he thought that it was MacCool's baby son.

Terrified at the possible size of the father, Finn Gall fled home to Staffa, destroying the causeway as he went.

The columns are spread along 275m (900ft) of coast. They reach as far as 150m (500ft) into the sea. Their regularity makes them appear to have been constructed by humans.

When viewed from above, the causeway resembles a street with regular paving stones - the columns fit together so exactly that it is difficult to insert a knife blade between them.

Most of the columns stand no higher than 6m (20ft), although some, such as the Giant's Organ, which resembles a church organ), can reach up to 12m (39ft).

Each individual column, which has the shape of a regular polygon, measures between 38cm (15in) and 50cm (20in) across. Most are hexagonal, but others can have four, five or as many as ten faces.

Formation of the Giant's Causeway

The Giant's Causeway is believed to be volcanic in origin.

About 50 million years ago, much of Northern Ireland and western Scotland became volcanically active.

Vents in the Earth's crust, such as those at Slemish Mountain, opened up frequently, pouring lava over the landscape to a depth of almost 160m (525ft).

When the lava cooled quickly, it solidified to form basalt, a tough, erosion-resistant rock. At Slemish, the basalt is piled up to a height of 441m (1,457ft) above sea level.

At the Giant's Causeway, however, the lava cooled slowly and steadily. As the upper levels lost their heat first, they shrank and cracked into regular patterns, much as mud cracks when it dries. The surface fissures gradually extended downward, splitting the entire mass of basalt rock into an array of upright columns of stone.

Softer surrounding rock has been eroded away at the coast, exposing the basalt columns to the air. The formation is thought to extend inland, beyond the spectacular line of cliffs known as the Giant's Cuffs and beneath the green landscape of County Antrim.

Isle of Staffa

The flat-topped Isle of Staffa, lying 120km (75mi) to the north of the Giant's Causeway, is also renowned for its six-sided columns.

Almost entirely composed of basalt, the island is ringed by cliffs of sheer, fluted columns and topped by a mop of spongy rock.

A huge cave, which penetrates 60m (200ft) into the island, was named Fingal's Cave, after the legendary giant Finn Gall, by Sir Joseph Banks, the English naturalist who accompanied Captain Cook on his 1768 voyage of exploration to the South Seas.

At low tide, the cave roof is about 18m (60ft) above the water. At high tide, or when the sea is whipped up by Atlantic storms, the water forced into the cave compresses the air and rhythmic sound, which resembles singing, can be heard.

Felix Mendelssohn, the composer, wrote his Fingal's Cave Overture in 1830, one year after visiting Staffa.