Scattered across this vast blue expanse are about 30 reefs and atolls, most of them wholly submerged or breaking the surface only at low tide. Where land does rise above the waves, it does so barely. The cays are low, flat and small; the territory's high point, on Cato Island, stands a mere six metres above the sea. The atolls range from a few kilometres across to the immense Lihou Reef, one of the largest atolls in the world by total area, whose lagoon spans a hundred kilometres while its actual dry land, eighteen tiny islets all told, adds up to less than a single square kilometre. To cross this territory is to sail for days between markers that are mostly underwater.

The single exception sits about 450 kilometres east of Cairns: Willis Island, a sliver of sand roughly 500 metres long. Since 1921 it has carried a weather station, established for one urgent reason, to give the Queensland coast advance warning of the cyclones that spin in from the Coral Sea. A staff of three or four observers lives there, the only permanent human presence in a territory the size of New South Wales. They were there in 2011 when Cyclone Yasi bore down, battening the buildings before a helicopter lifted them clear, and the instruments recorded the storm's fury until they failed. Across the rest of the territory, automatic stations and the occasional lighthouse stand watch on the empty reefs, machines keeping vigil where no one lives.

What the islands lack in people they make up for in wings and flippers. The forested cays of the Coringa-Herald and Lihou groups are among the most important seabird rookeries in the Coral Sea, home to breeding colonies of more than a dozen species, terns and boobies and tropicbirds wheeling in their thousands above the sand. The waters around them shelter six species of marine turtle. The threatened green turtle hauls out to nest on the cays of the Coringa-Herald cluster and on eleven small islands of Lihou Reef, dragging itself up the beach in the dark to dig and lay, as its kind has done here for longer than there have been ships to wreck nearby. Much of the territory is now protected reserve, knitted together into a wetland of international significance.

These same beautiful, low-lying reefs have always been deadly to ships. The territory was first charted in 1803, the very year HMS Porpoise drove onto its coral and gave Wreck Reefs its name, stranding Matthew Flinders and ninety-odd others on a sandbank within these waters. In the 1870s and 1880s prospectors briefly mined the cays for guano, but with no reliable fresh water no one could stay. The Royal Australian Navy patrols the territory now, and divers come for some of the clearest water and most dramatic walls in the world, the sheer drop-offs of Osprey Reef among them. It remains one of the least-visited corners of Australia, a near-empty ocean dotted with sand, where the loudest sound is the colony of seabirds and the surf breaking on reefs that have swallowed more than one ship whole.