Trinity House completed South Stack Lighthouse in 1809. The lamp tower is 28 metres tall and the whole complex covers seven acres. For its first nineteen years the only way to cross from the mainland to the stack was in a wicker basket suspended from a hemp cable - the keepers and supplies swung across a 30-metre gap of churning water. In 1828 an iron suspension bridge replaced the basket; a steel cable truss bridge replaced it in 1964, and an aluminium bridge replaced that in 1997 after the previous one had been closed for safety reasons. The original lamp was replaced by an Argand oil burner, then by paraffin vapour, then by electricity. The light shines at 60 metres above mean sea level and can be seen 28 nautical miles offshore. A foghorn was added in 1873 - the kind of sound that, if you have heard it, you do not forget.

The Geological Society of London picked South Stack as one of the top 100 geosites in the United Kingdom, and the reason is visible from the staircase down to the bridge. The rock here is the South Stack Formation: sandstones and shales laid down on an ancient sea floor about 580 million years ago, then crumpled by tectonic forces into folds within folds within folds. From a few metres away the cliffs read as horizontal stripes. Up close, the stripes reveal themselves as wild curves and switchbacks, sediment beds bent back on themselves like ribbon. The thick beds of pure white Holyhead Quartzite that cap Holyhead Mountain above sit on top of all this folded chaos, evidence of a separate, later layer of geological violence.

An estimated 8,000 seabirds nest on the South Stack cliffs each breeding season. The RSPB runs a visitor centre at Elin's Tower on the mainland, with a bird hide trained directly on the loudest face. In good binocular weather you can pick out razorbills with their thick parrot-bill beaks; common guillemots packed shoulder to shoulder on impossibly narrow ledges; black-legged kittiwakes glued to almost vertical rock with mud nests; the occasional puffin; and, working the air above the colony with their distinctive red bills and feet, choughs - the rare crow species that has become one of the symbols of Welsh cliffs. Peregrine falcons hunt here. Below, in the water, harbour porpoises, grey seals, and Risso's dolphins are regular visitors. The endemic South Stack fleawort, a small yellow daisy-relative, grows only on these cliffs and nowhere else on Earth.

In August 1975, the photographer Graham Hughes climbed down to a slope directly below the footbridge with the model Jerry Hall and shot the cover of Roxy Music's Siren album. Hall, glistening with paint to look like a stranded mermaid, lay on the rocks while the lighthouse stood out of frame above. The pictures made South Stack briefly famous to a generation that had never heard of Trinity House. In 2021 the Welsh Government granted consent for Morlais, a tidal energy project in the waters between South Stack and the Skerries, intended to be one of the largest in Europe. The onshore grid connection was completed in 2023. The RSPB and others have warned that the project's underwater turbines could devastate the razorbill and guillemot colonies that have made this cliff their home for centuries. Whether the lighthouse keeps its sea unchanged for the next two hundred years, after surviving the last two, is now an open question.