Trunk Bay is an eight-hundred-metre crescent of white sand on the north shore of St John, in the U.S. Virgin Islands, and there is a marked trail on its seabed. A line of plaques bolted to the reef makes up the first underwater trail in the American park system, in water shallow enough for you to read on a held breath. The panels put in during 2021 do not simply name fish. They say plainly that the reef under you is struggling. Above the surface none of that shows. The sand curves between two green headlands with a lifeguard stand and sea grape for shade, two rocky cays sit a short swim out, and the water is so clear that ripples on the bottom keep their edges three metres down. Kayaks take you out along the line between the cays, where the pale lagoon drops into jade and eagle rays cross the deeper channel. Most people know the bay from the high bend on Northshore Road before they ever stand on it.
The trouble arrived in 2019. Stony coral tissue loss disease attacks more than twenty kinds of hard coral here and can strip a colony bare within weeks. Divers treat it the way a doctor treats an infection, smearing a paste laced with amoxicillin onto the white lesions one by one and raising rescued fragments in nurseries. It buys time for individual corals. It does not cure a reef. The territory has also outlawed three sunscreen chemicals outright, not just selling them but carrying them onto the beach, because they wash off swimmers and deform coral larvae. Away from the coral the edges of the bay are seagrass, and green turtles come up to crop it, surfacing for a breath a few metres from you. The cruise excursions from St Thomas fill the sand by mid-morning and are gone by three. From June the hawksbills haul out to nest, and St John logged a record 44 nests in 2025, 37 of them hawksbill. Every one of those females came back to lay on the beach where she hatched.