Strangford Lough was carved at the end of the last ice age, when glaciers retreating from County Down left behind a basin that the sea promptly filled. Most of the lough is less than ten metres deep, but the central channel can plunge to fifty. The Ards Peninsula almost completely encloses it, leaving only the Narrows at the southeastern edge as a connection to the Irish Sea. Inside, the lough holds at least seventy named islands and countless smaller islets - locally called pladdies - along with mudflats, salt marshes and tidal creeks. The previous Irish name for this water was Loch Cuan, meaning lough of the harbours. The shift to Strangford, originally just the channel name, only spread to the whole body of water in the seventeenth century. Both names are accurate.

Strangford was designated Northern Ireland's first Marine Conservation Zone in 2013 and is a Special Area of Conservation, an Important Bird Area and a Ramsar wetland. Each autumn, around three-quarters of the world population of pale-bellied brent geese arrive here to feed on the eelgrass beds - sometimes 15,000 birds at once, having flown south from breeding grounds in Arctic Canada. The lough holds Ireland's most important breeding site for common seals. Basking sharks cruise through the Narrows in summer. The Castle Espie wetland reserve, on the western shore near Comber, hosts the Wildfowl and Wetlands Trust's collection and serves as a stopover for migrating ducks and waders. Underwater, the lough is just as rich. More than 2,000 marine species have been recorded here, including extensive beds of maerl - a calcareous red algae that builds slow-growing reef-like structures - and the carpet sea squirt, an invasive species first detected in 2012 that conservation managers are still trying to contain.

The same currents that make Strangford remarkable for wildlife make it irresistible to engineers. In 2008 the lough became home to SeaGen, the world's first commercial tidal stream generator. The 1.2-megawatt turbine fed electricity to the grid for eleven years, with monitoring showing minimal impact on marine mammals. A second device called Evopod, a one-tenth scale floating prototype monitored by Queen's University Belfast, was tested near the Portaferry ferry landing in 2008. SeaGen was decommissioned in stages between 2016 and 2019, but the lough's reputation as a test bed for tidal energy has not faded. Strangford Lough was also, far earlier, the site of the world's oldest known tide mill - the Nendrum Monastery mill, where archaeologists have excavated timber pilings dating from 619 CE. Twelve hundred years separate the monks who built that wheel from the engineers who installed SeaGen, but both were answering the same question.

A car ferry has connected Portaferry and Strangford across the Narrows for nearly four centuries, without a meaningful break. The crossing takes eight minutes; the alternative road journey around the lough is 47 miles and takes about an hour and a half, which explains why locals view the ferry less as transport and more as essential infrastructure. The service is subsidised and operates at a loss of more than a million pounds a year. It still runs every half hour from 7:30 AM to 10:30 PM. Towns and villages around the lough's shoreline - Killyleagh, Comber, Newtownards, Portaferry, Strangford - each have their own character, but all of them measure distance partly in ferry minutes. In July 2016, Strangford hosted the Skiffie Worlds, an international coastal-rowing championship that brought clubs from Scotland, England, the Netherlands, the United States, Canada, Tasmania and several other countries. Fifty teams raced a 2-kilometre course at Delamont Country Park, with the lough's flat morning water and its dramatic afternoon currents acting as the test conditions.