Foreign arrivals talk about the diving, the motorbikes and the emptiness. Vietnamese arrivals talk about Hàng Dương, the cemetery on the northern edge of town, where the queue for one particular grave forms after dark. The two crowds share an airport and almost nothing else. Nobody should need telling twice about the etiquette: bare shoulders and short trousers are not welcome among the graves, and riding a motorbike across them is unthinkable. What changed the volume was a boat. In 2016 a fast ferry opened from Trần Đề, in Sóc Trăng, cutting the crossing to roughly three hours and putting the island within a weekend of the entire Mekong Delta. A hydrofoil from Vũng Tàu followed in February 2019, four to five hours each way. Domestic pilgrimage, not international tourism, is what filled the hotels.

Côn Sơn town occupies a crescent-shaped valley between green hills and a long grey-blue bay, and it is small enough to cross on foot before the sun gets serious. You do not so much visit the prison here as fail to avoid it. The complex reached its greatest extent in 1973 with seven camps; five are open, and tickets are sold at Camp 3, in the middle of town. Beyond that, the fragments are simply part of the road network. Sở Lò Vôi, a lime kiln worked by convicts, sits on the way to the airport. The Cow Pens and the Pig Sty, named for exactly what they resemble, stand on either side of Võ Thị Sáu street. Sở Muối crumbles beside the coast road to Bến Đầm, near a Buddhist temple gate. Above the town, reached by a steep and easily missed trail, lie the ruins of Sở Rẫy, a colonial plantation whose labour force was the prison population, now a viewpoint with one of the best panoramas on the island.

Remoteness here is not a marketing adjective. The island has a single filling station, and it opens from 07:00 to 10:30 and again from 14:00 to 16:00, which is the kind of detail that reorganises a day. Rent a bike with a full tank. Helmets are legally required and practically essential, because the medical facilities are rudimentary and evacuation to the mainland is slow, complicated and expensive — dive operators here treat insurance as non-negotiable for good reason. Against all that, the riding is a pleasure: little traffic, gentle gradients, and roads that climb toward Thánh Giá mountain, at 577 metres the high point of the archipelago. Crime is close to nonexistent. Locals leave bicycles and motorbikes outside overnight, which in Vietnam is remarkable enough to be a standing joke.

Roughly four-fifths of the archipelago's land is national park, and it shows the moment you leave the tarmac. The short stone path down to Ông Đụng beach passes through genuine forest, and the macaques that live along it belong to a subspecies found only on these islands. The Côn Đảo black giant squirrel is the other local celebrity, glimpsed more often than photographed. Offshore, most of the surrounding sea is a no-take marine protected area, and the reefs are correspondingly intact. Snorkelling from the main island is reliably good in only one stretch and only from May to October; the better water lies off Bảy Cạnh and Hòn Cau, both day trips by boat. Divers get the widest choice — with sixteen islands to shelter behind, there is usually somewhere with visibility no matter the season.

Fewer than fourteen thousand people live here, and the arithmetic of a tourism boom on a small island rarely favours them. Much of the visitor money concentrates in the larger hotels; the market, the local kitchens and the independent bike hires are where it reaches households. There are practical rhythms to learn, too. Mosquito season runs February to April and there is no malaria, but the sand flies of January to May leave bites that itch for days and infect easily if scratched. Fresh water is precious and the island's plastic problem is visible; refilling a bottle costs a fraction of buying one. Côn Đảo is still, for now, a place where you can walk a kilometre of sand and meet nobody. That sentence has an expiry date, and everyone on the island knows it.