The story did not start in Strangford. In 1994, a small UK engineering company called Marine Current Turbines demonstrated a 15-kilowatt prototype tidal generator in Loch Linnhe, on the west coast of Scotland. Nine years later, in May 2003, they scaled up to a 300-kilowatt machine off Lynmouth in North Devon. Strangford was the third step, but the first one that mattered commercially. The chosen site - the narrow channel between Strangford and Portaferry, where the tides force their way in and out of a 150-square-kilometre lough through a gap less than a mile wide - happens to be one of the few places in the British Isles where tidal current speeds reach four metres per second. There was also a historical resonance. Strangford Lough is also the site of the world's first known tide mill, the Nendrum Monastery mill, where archaeologists have excavated timber pilings dating from 619 CE. Both ends of a 1,200-year experiment in tidal power happened in the same body of water.

SeaGen's design was elegantly straightforward. Two 16-metre axial-flow rotors, each sweeping 200 square metres, sat at the ends of a horizontal crossbeam mounted to a vertical pile driven into the seabed. Each rotor drove a generator through a three-stage gearbox - two planetary stages and one spur stage - that multiplied the rotor speed by 69.8 times. The rotors themselves turned at a leisurely 12 rpm, slow enough to give seals and basking sharks time to swim around them. The whole crossbeam could be raised above the water for maintenance, which made the machine famously easy to service. The first months were not flawless. During commissioning, a software error damaged the blades of one of the rotors, and the system limped along at half power until autumn 2008. Full operation was finally achieved on December 18, 2008, a date the engineering team marked as the real start of the industry.

From the beginning, SeaGen was treated as much as a science experiment as a power station. Its operating licence required a five-year environmental monitoring programme, and Queen's University Belfast led the research. The team watched the lough's resident seals, harbour porpoises, basking sharks and migrant seabirds with cameras and sonar. After three years of monitoring, the conclusion was striking and a little anticlimactic: there were no significant impacts on marine mammals. Seals seemed to avoid the device when it was running, or transit past it at slack water when the rotors were still. Some marine birds shifted slightly within the Narrows, but the overall populations did not change. For the tidal-power industry, this was the result that mattered most - proof that you could put a rotating machine into a tidal channel and not measurably harm the wildlife that depended on it.

Marine Current Turbines was sold to Siemens in 2012, and Siemens in turn sold the company to Atlantis Resources (now SIMEC Atlantis Energy) in 2015. Plans existed for multi-rotor systems, including six-rotor configurations capable of generating eight megawatts at a single site. None were built. The Strangford machine was shut down in 2016, and removal proceeded in stages until summer 2019, when SIMEC Atlantis announced the world's first decommissioning of a commercial tidal energy device. It was a quiet achievement that mattered. Anyone who wants to build tidal infrastructure now knows that the assets can come back out cleanly when their service life ends - a question the offshore wind industry is only beginning to face. SeaGen exists now mainly as data: eleven years of operational records, monitoring reports, and a single set of timber pilings in Strangford mud where a turbine used to spin.