Po Toi sits about 9 kilometres southeast of Stanley, the farthest inhabited island in Hong Kong's chain. The lighthouse — officially Lighthouse 126, also called Nam Kok Tsui after the peninsula it crowns — was constructed and lit in 1970. It is a functional, unpretentious structure: a concrete equipment building with a gallery and light mounted at the top. No elegance to speak of. Its job is to be visible, not beautiful. And it does that job at the edge of the chart, where the land finally lets go.

Reaching the lighthouse requires hiking the Po Toi Country Trail, a path that winds across the island's rugged terrain past ancient rock carvings believed to date back over 3,000 years. The trail eventually reaches the southeast peninsula and the stairway with its railing — a thin line of human order imposed on a wild, wind-exposed headland. On clear days, the view from the top sweeps across open ocean. Freighters and container ships pass in the middle distance, bound for or departing from Kwai Tsing Container Terminals to the north. The lighthouse earns its existence with every one of them.

Po Toi is one of Hong Kong's few islands genuinely remote enough to feel isolated. A small fishing community survives there, and kaido ferries make the trip from Aberdeen and Stanley on weekends — but on weekdays the island is quiet. The lighthouse operates automatically; no keeper lives there. Hikers make the trek, birdwatchers come during migration season when Po Toi lies directly on one of Asia's major flyways, and occasionally sailors take note of its beam. For all of Hong Kong's vertical, neon-lit density, this small concrete light at the territory's southernmost reach is about as far from all that as it is possible to be.

Lighthouses have always marked thresholds — the line between known and unknown, between safe water and danger. Po Toi Lighthouse sits at a particular kind of threshold: the boundary of one of the world's most consequential port cities. Ships that clear this point are entering or leaving the orbit of Hong Kong harbour, one of the busiest deep-water ports on Earth. From a helicopter above, the lighthouse reads as a small white mark at the island's pointed southern tip — easy to miss against the scale of the sea. From the deck of a vessel navigating the eastern approaches at night, it would be the first thing a navigator looks for.