Sarn Badrig is one of three long, low submarine ridges that run out into Cardigan Bay from the Welsh shore - linear features of glacial deposit that became, in Welsh folklore, the embankments of the drowned kingdom of Cantre'r Gwaelod. For sailors approaching the coast in the era before reliable charts, they were a nightmare. The bay looks open and deep. The sarns lie just below the surface in places, just above it in others, running for miles out into water that should by every reasonable measure be safe. The Welsh word sarn means causeway, and old folklore claimed Saint Patrick had paved them. They were the graveyard of so many ships that any wreck found in the right place could plausibly be almost anything.

In 2000, two local divers - Tony Iles and his daughter Helen - found something on the seabed with a magnetometer. They dived it. They found wooden frames reinforced with iron, a hull stretching more than 150 feet, and pins of yellow metal. Yellow metal is the bright copper-zinc alloy called Muntz Metal, used to sheathe wooden hulls against marine borers. They reported the find to Cadw, the Welsh heritage body. Information from a third party suggested the wreck was the Diamond - and further, that the Diamond had been the oldest known example of a composite hull, wood reinforced with iron, an important early step toward the iron ships of the later 19th century. This made the wreck nationally significant. The Archaeological Diving Unit, operating under the Protection of Wrecks Act 1973, surveyed the site with side-scan sonar and a remotely operated vehicle. On 1 April 2002, Cadw declared the wreck to be the Diamond and designated it a protected site - the first such designation made by the newly devolved Welsh administration. Recreational divers were banned from the location.

It almost certainly is not the Diamond. Ian Cundy of the Malvern Archaeological Diving Unit was licensed to survey the site in July 2002, and what he found did not match. The Diamond was 120 feet long; the wreck is closer to 160 feet. The Diamond was a standard white-oak-and-locust-wood vessel with copper sheathing fastened with copper tacks - the composite hull story turned out to be entirely false. The Welsh wreck's hull samples did not match the Diamond's documented timbers. Worse, the yellow metal scattered on the seabed is stamped Muntz Metal, an alloy patented by George Frederick Muntz in 1832 - seven years after the Diamond sank. The wood in the main ribs, when professionally dated by tree-rings in 2006, was still growing in 1825 and was not felled until around 1840. The wreck below the waves is a real wreck. It is just not the Diamond, which lies somewhere else on this graveyard of a bay, still uncatalogued, still keeping its name.

The unknown ship that the Welsh government designated as the Diamond is still protected. The Diamond's actual resting place, somewhere on or near Sarn Badrig, is still unmarked. The bay has eaten too many vessels for any one of them to be easily picked out from the rest. The reef has not changed. It still runs miles out into water that looks open. It still catches what runs over it in fog or darkness or in a January gale on the run home from New York. Captain Macy lived. So did most of his passengers - the calm waters and visible spars after the wreck made rescue possible, which is the small mercy of striking the sarn in the right kind of weather. The international mail did not arrive. The cotton, potash and apples went to the bottom. And the precedent of an early scheduled trans-Atlantic packet service stayed afloat without her, because shipping companies had already learned the lesson: regular runs, fixed schedules, paying passengers - the ocean liner was coming, whether the Diamond made it or not.