The Cornish name for the Runnel Stone means stone abounding in seals. Until 1923 it lived up to the description: a black knuckle of granite that appeared above the surface at low water and provided a basking platform for the local grey seal colony. In 1923 a steamship called the City of Westminster ran full into it. The collision sheared the pinnacle off below the waterline. The stone has not shown itself since. It is still there, one mile south of Gwennap Head, eight feet below the surface at low water, and the ships kept coming. Between 1880 and 1923, identified records show more than thirty steamships lost in the immediate area. The unidentified count, the casual wreckings that never made the newspapers, would be larger.

Trinity House first put a marker on Gwennap Head in 1821, a bare wrought-iron pole that the sea carried away within a season. James Walker, the corporation's chief engineer, designed the replacement. Work began in 1841 and proved, in the surveyors' words, of great difficulty and danger. Only a small portion of the rock protruded from the water. Boats could land only at spring tides, and even then the sea was seldom calm enough to admit a footing. The completed daymarks are simple in concept and beautiful in execution. The principle is called a transit, a line of sight running through two known points. If you can see only the seaward red cone, you are too far south, in clear water. If the black-and-white inland cone shows alongside it, you are still safe. When the cones perfectly overlap, you are immediately above the stone. It is an analog navigation aid that requires no electronics, no batteries, and no maintenance beyond paint.

A buoy has marked the position of the Runnel Stone since the 1870s. The most beloved version had three voices. A flashing light for the eye, a bell that rang as the buoy rocked, and a whistle set in a tube that emitted a long mournful note when there was a swell running. The whistle was the famous one. Walkers on the cliffs at Gwennap Head heard it drifting in from a mile offshore in foggy weather, an unhurried hooting sound that carried unmistakably through the mist. People came for the hooting alone. In May 2012 Trinity House replaced the buoy with a larger model, and the bell was retired in favour of a whistle to replace it. The cliffs are quieter now.

On 29 January 2016 the area was designated the Runnel Stone Marine Conservation Zone, twenty square kilometres of reef, sandy seabed, and crumpled steamships protected from bottom trawling and aggregate dredging. The MCZ exists because the stone has been so thoroughly successful at sinking ships. Divers go down at slack water, about ninety minutes before high tide at Newlyn, into visibility that can reach twenty metres on a good summer day. They find boilers and prop shafts and entire bow sections from a dozen different vessels, encrusted now with dead-men's fingers and jewel anemones in colours that nobody ever named. Sea fans grow in the current. The seals, the original tenants, are still around. They watch the divers from a safe distance and, occasionally, follow them home.