There are four of them. By far the largest is Muckle Skerry - muckle meaning big in the Scots vernacular - a low platform of grass and rock perhaps 35 hectares across, the only island substantial enough to have ever supported a human presence. To its south, in a line running west to east, sit the smaller three: Little Skerry, Louther Skerry and Clettack Skerry. Together they form a rough archipelago strung across the southern half of the Pentland Firth, four miles northeast of Duncansby Head and about three miles south of South Ronaldsay. The waters between them are shoal and treacherous. The currents here can run at twelve knots when the tide is in full flood, fast enough to overwhelm small craft and make even big ones thoughtful.

Muckle Skerry has a pair of lighthouses, both lit in 1794. They were among the very first of the Northern Lighthouse Board's installations, built when the Pentland Firth was becoming busier with North Sea trade and the death toll from wrecks was climbing fast. Two towers rather than one - one at each end of the island, with the alignment of the two lights letting mariners triangulate their position in the dark. The keepers lived on Muckle Skerry in cottages between the lights, supplied by sea, cut off for weeks at a time when winter gales made landing impossible. The lights were automated decades ago. The keepers' cottages remain, weathered shells, slowly being reclaimed by the wind.

BirdLife International has designated the skerries part of the Pentland Firth Islands Important Bird Area - a recognition that these little rocks support breeding populations of seabirds significant on a national, even European scale. Common guillemots, razorbills, kittiwakes, fulmars and Atlantic puffins arrive each spring, the puffins digging burrows into the turf above the cliffs, the auks crowding the ledges. The cliffs come alive with their noise and smell from April through August. In winter the colonies disperse to sea and the skerries return to silence, broken only by the seals hauled out on the rocks and the wind that never quite stops.

The Pentland Firth is one of the few stretches of water in the world where the tides genuinely do create standing waves and overfalls big enough to threaten shipping. The Atlantic floods then drains the entire North Sea through this narrow gap twice a day, and the skerries sit right in the middle of it. Local sailors have names for the dangerous spots - the Merry Men of Mey, the Swelkie - and stories about what happens to those who ignore them. The MV Priscilla, a general cargo vessel, ran aground here in July 2018 in calm conditions; the Marine Accident Investigation Branch's subsequent report read like a lesson in how easily routine seamanship can become emergency rescue when the tide takes charge. Nobody died. The ship was salvaged. The skerries, predictably, remained exactly where they had always been.