Ha Pak Nai's 6-kilometre shoreline carries one of Hong Kong's most intact wetland ecosystems. Mangroves root in the intertidal zone, their exposed root systems filtering the water and providing nursery habitat for dozens of species. The mudflats are a designated Site of Special Scientific Interest — a classification that reflects the richness of what lives here. The area was historically an oyster bed, and remnants of those shells still crust the flats in places, a record of an earlier economy written in calcium. Among the crab species found here — Perisesarma bidens, Ilyoplax tansuiensis, Uca arcuata, Sesarma sinensis — the horseshoe crab is the most closely watched. Not an actual crab but a chelicerate more closely related to spiders and scorpions, the horseshoe crab's lineage reaches back 450 million years. The copper in its blood turns blue when exposed to oxygen, giving these ancient animals an otherworldly quality that matches the strangeness of the flats they inhabit.

Beneath the visible wetland, the ground holds older histories. In May 1997, archaeologists from the Antiquities and Monuments Office excavated the Chen's homeland relic — a sand dune with two culture levels, containing four postholes, nine stone stoves, three kilns, and one grave. The excavation produced the first examples in Hong Kong of what are called sun and moon utensils, objects whose significance connects the site to ancient ritual practices along the Pearl River coast. Nearby, at No. 55 Ha Pak Nai, a fortified structure carries a different kind of weight. Tang Yam-nam, a core member of the Hsing Chung Hui — the Revive China Society founded by Sun Yat-sen — built the structure around 1910. Hsing Chung Hui members used Lower Pak Nai as a base for preparations to overthrow the Qing dynasty, and the structure served as a surveillance point across the Deep Bay coast. The 1911 revolution that ended the Qing dynasty is one of the defining events of modern Chinese history, and this fortified building is the only physical evidence connecting that movement directly to this coastline. It is now a declared monument.

Ha Pak Nai means Lower White Mud in Cantonese. It is known — and this is not contested — as one of the best places in Hong Kong to watch the sun go down. The bay faces due west, toward Shenzhen and the Pearl River estuary beyond. On clear evenings the sky above Deep Bay takes on colours that the concrete and glass of Central or Mong Kok never allow a viewer to fully appreciate: amber pooling at the horizon, then rose, then a purple that fades slowly into the bay. Photographers arrive early to claim positions along the shore. Dating couples follow. The 5-kilometre cycling route along Nim Wan Road has become a local institution, the kind of route that appears in lifestyle articles about Hong Kong's outdoor spaces. The mudflats that edge the shore reflect the changing light with unusual depth. Oyster shells catch it. The silhouettes of fishermen sometimes cross it. The horseshoe crabs, indifferent to it all, continue their ancient business in the shallows.

Not long ago, Pak Nai faced the kind of development pressure that has reshaped most of the New Territories' coastline. A private proposal for development on the wetland — including, at one point, a golf course — generated significant opposition from conservationists, local villagers, and the public. The proposal was eventually dropped. The Site of Special Scientific Interest designation and the declared monument at No. 55 provide some protection. Reaching Pak Nai requires effort: it sits at the far western end of the New Territories, accessible by minibus number 33 from Tai Fung Street in Yuen Long, or by taxi. That relative inaccessibility has contributed to its survival. The mudflats do not advertise themselves. They simply persist — as the horseshoe crabs persist, as the mangroves persist, as the memory of a revolution that swept away the last Chinese dynasty persists in a small fortified building by the water.