Peninnis Lighthouse does not look like a classic English lighthouse. Most of them are stout masonry towers of dressed stone, the legacy of Trinity House's 18th- and 19th-century building programmes. This one is industrial. Its base is an open lattice of black-painted steel, the kind of skeletal frame you would expect on a Victorian pier or an early radio mast. Above the lattice sits a white gallery and then a black-domed top. The whole structure is circular and 45 feet tall. The design reflects what was practical in 1911 rather than what would have been romantic. Steel had become the right material for an exposed Atlantic headland; the lattice let storm winds pass through rather than push against; and the construction was much faster than the stone alternatives.

When first lit in 1911, the lamp burned incandescent oil gas. Four pressurised tanks of oil gas, holding a combined 700 cubic feet, sat in an adjacent one-storey building, replenished by the local Trinity House supply vessel. The light was classed as semi-watched, requiring no full-time keeper. A clockwork-driven third-order Fresnel lens rotated the beam in a white flash every 20 seconds. The range was 16 nautical miles. In 1922 the system was converted to automatic acetylene, with the same gas driving both the lamp and the lens rotation. In 1992 the lighthouse went over to mains electricity. In late 2011, on its centenary, Trinity House replaced the rotating optic with a single-tier LED lantern mounted on the exterior rail. The Fresnel lens still sits inside, but no longer turns.

The 2010 Trinity House Aids to Navigation review concluded that satellite navigation and electronic charting had reduced the need for high-intensity coastal lights. Peninnis was downgraded as part of that review. Its old 16 nautical mile range was cut to 9 nautical miles. The reduction is a quiet admission of how navigation has changed. A century earlier, this beam had to be visible to a sailor relying entirely on dead reckoning and a sighting through driving spray. Today's vessels have GPS, AIS and digital charts; the lighthouse's role has shrunk to a supplemental visual aid for the last few miles into St Mary's Sound. Yet it still flashes white every 20 seconds, on the same headland, doing the same job in a different century.

St Mary's Sound is the eastern entry into the great natural anchorage at the centre of the Isles of Scilly. It runs between Peninnis Head on the south, the western islands of St Agnes and Annet to the south-west, and the rocks of the Spanish Ledges and the Newfoundland reefs to the south-east. A vessel inbound for Hugh Town harbour, the principal port of the archipelago, threads up the Sound with Peninnis to starboard. The lighthouse marks the corner of the approach. The 1707 fleet that sank with Sir Cloudesley Shovell was nowhere near this particular point, but those wrecks are the reason Trinity House cared so much about this coast. Every light along this archipelago is an admission that 1,450 men died, and someone owed it to history not to let it happen again.

The lighthouse was Grade II listed in 1992, the same year it was electrified. Listed buildings in England are normally chosen for architectural or historic interest. Peninnis is interesting on both counts: as an early-20th-century example of a steel-lattice lighthouse, and as a working aid to navigation continuously in service since 1911. Trinity House still owns and operates it. Visitors can walk to the base on a footpath from Hugh Town, picking their way through the natural granite cairns that crowd the headland. From a distance the lighthouse looks too thin to be useful. Up close, the steel is heavy industrial gauge, painted and re-painted in alternating black and white, and the structure stands solid in winds that knock walkers off their feet.

Coordinates: 49.9046°N, 6.3035°W. The lighthouse occupies the southern tip of Peninnis Head, the southernmost point of St Mary's. From 2,000 feet, the black lattice silhouettes against the pale granite of the headland; from 1,000 feet you can pick out the white gallery and dome. St Mary's Airport (EGHE) sits 1.5 kilometres to the north-east. The lighthouse is one of the principal visual landmarks of the southern Scilly approach. Easily paired with views of the Peninnis Head cairns themselves and St Mary's Sound to the west.