The reef began forming 20 million years ago, though the current structure dates to the end of the last ice age, about 8,000 years ago. It grows atop the drowned continental shelf, where warm, shallow, sunlit water provides ideal conditions for coral polyps - tiny animals related to jellyfish that build limestone skeletons. Over millennia, generations of polyps built upon the skeletons of their ancestors, creating the massive formations divers explore today. The reef is not uniform: coral cays and continental islands interrupt the blue; shallow lagoons give way to reef walls dropping into deep water; hard corals share territory with soft fans and sponges. Each section harbors different communities of fish and invertebrates, making no two dives quite alike.

Cairns serves as the reef's primary gateway, with day boats and liveaboard vessels departing daily for reef platforms and pontoons. Port Douglas, to the north, offers similar access with a more upscale atmosphere. From Airlie Beach, boats head to the Whitsunday Islands and their fringing reefs. Townsville provides access to the SS Yongala wreck, where a 1911 shipwreck now hosts one of Australia's most spectacular dive sites. The reef approaches closest to shore near Cape Tribulation, where rainforest meets sand beach and reef lies just offshore - two World Heritage sites in a single view. Everywhere, tour operators range from backpacker-friendly budget boats to luxury catamaran experiences, with options for snorkelers and certified divers alike.

The reef reveals itself differently to snorkelers and divers. Snorkelers skim the surface above coral bommies, watching clownfish dart among anemones and parrotfish crunch coral. Divers descend to walls where sharks patrol and moray eels peer from crevices. Night dives reveal a different reef: coral polyps extend to feed, octopuses hunt, flashlight fish blink bioluminescence. The annual coral spawning, typically occurring over a few nights in November, sees reefs explode in slow motion as millions of coral colonies release eggs and sperm simultaneously - a synchronized reproduction event covering thousands of square kilometers. For divers who time their visit precisely, it's one of nature's most spectacular phenomena.

The reef that exists today is not the reef of a generation ago. Mass bleaching events in 2016, 2017, 2020, and 2022 killed coral across vast stretches of the system. Bleaching occurs when heat stress causes corals to expel the symbiotic algae that give them color and nutrition; prolonged bleaching leads to starvation and death. Climate change has raised sea temperatures to the point where bleaching events now occur faster than reefs can recover. Crown-of-thorns starfish outbreaks, agricultural runoff, and cyclone damage compound the stress. Yet coral shows surprising resilience in some areas, and scientists are experimenting with heat-resistant coral varieties, larval reseeding, and cloud brightening to cool reef waters. The reef's future depends on global climate action, but visiting responsibly - choosing reef-safe sunscreen, supporting conservation-minded operators - contributes to the effort.

Day trips to the reef run about two hours each way from Cairns, less from Port Douglas. Most operators visit pontoons moored over reef sites, providing access via snorkeling, semi-submersible boats, and introductory dives. Liveaboard trips range from overnight to week-long expeditions reaching the remote ribbon reefs and Coral Sea. Water temperatures range from 24°C in winter to 29°C in summer - warm enough for extended swimming without wetsuits, though many wear them for sun and stinger protection. Box jellyfish season runs October through May, primarily near beaches rather than outer reefs, but most operators provide stinger suits regardless. The Queensland sun is fierce; reef trips involve hours of exposure, making high-SPF reef-safe sunscreen essential.