The Cod Hole sits at the northern end of the Great Barrier Reef, on the outer edge of Ribbon Reef Number 10, near the channel once called Cormorant Pass. The ribbon reefs are a chain of long, narrow coral ridges that run along the edge of the continental shelf, walling the sheltered lagoon off from the open Coral Sea. The dive itself is a coral garden in clear blue water, with sandy lanes running between the formations where the cod rest and patrol. The reef wall drops away nearby into deep ocean, so the site sits at a meeting point of shallow coral and open sea, which is part of why the big fish gather here. Visibility is often superb and the water warm. Framed by the remote reefs of the far north, reachable only by liveaboard boat or a long day trip, it is about as wild and unspoiled as reef diving gets.

Potato cod are a kind of grouper, and they grow to a startling size, reaching one and a half to two metres long and well over a hundred kilograms, roughly the dimensions of a household refrigerator with fins. They are named for the blotchy, potato-like markings scattered across their pale, heavy bodies. A dozen or so resident cod live around the site, each a recognisable individual to the dive guides who visit them year after year. They are extraordinarily bold. A cod will swim straight up to a diver, hover at eye level, and then follow along the reef with the frank, unhurried curiosity of an animal that has never learned to be afraid of people. Encountering one is the closest most divers come to meeting a large wild animal that simply does not flee. It can be unnerving for the first few seconds, when instinct insists that something this big should not be this close, and then it becomes the kind of memory that stays with you for the rest of your life.

The Cod Hole owes its fame to two people. The pioneering underwater filmmakers Ron and Valerie Taylor discovered the site in 1971 and began visiting regularly, and their footage of the great approachable cod made it world-renowned. Divers have hand-fed the fish for decades, which is exactly why the cod are so habituated, and exactly why the practice became controversial. As crowds grew, so did the worry. Researchers found that the average number of potato cod seen per dive nearly halved between 1992 and 1998, and the heavily visited, deliberately fed site became a flashpoint in the wider debate over how to manage tourism on the reef without loving it to death.

That debate matters more now than ever. The same northern reefs that make the Cod Hole spectacular are on the front line of a warming ocean, hit by repeated mass coral-bleaching events that have damaged huge stretches of the Great Barrier Reef. The potato cod still cruise their sandy lanes, but the coral garden around them is part of an ecosystem under real strain. To dive the Cod Hole today is to witness something genuinely rare, a place where giant wild fish will swim up to greet you, and to feel, at the same time, how precarious such wonders have become. Few encounters in the ocean leave you so exhilarated and so determined that they should not be lost.