Christmas Island

Christmas Island is an external Australian territory sitting on the rim of the Java (Sunda) Trench in the eastern Indian Ocean, roughly 350 km south of Java and over 1,500 km from the Australian mainland — reached by limited Virgin Australia flights from Perth, so trips need planning around a thin schedule. Geologically an oceanic seamount, the island has only a narrow fringing reef before the seafloor falls away into vertical walls that drop several kilometres; most walls begin only about 20 m from the shoreline, putting world-class wall diving within easy reach of shore. Visibility is exceptional, often 30–50 m, water temperatures sit around 26–29°C year-round, and the reef supports more than 575 fish species and over 100 species of hard coral. Because the island lies on the biogeographic border where Indian and Pacific Ocean faunas meet, it is a globally significant marine 'suture zone': at least 11 hybrid reef-fish pairs (surgeonfish, angelfish, butterflyfish, triggerfish, wrasse and pufferfish) have been recorded here — more than anywhere else on Earth — alongside endemics such as the Cocos angelfish. Every wet season (roughly November–March/April) whale sharks and manta rays gather to feed on the billions of larvae released into the sea during the spectacular red crab spawning, a terrestrial migration of tens of millions of crabs that times with the marine season. Diving is generally possible year-round — Flying Fish Cove offers protected shore diving even in the wet — but conditions and whale-shark sightings vary, and there are no guarantees.

• Christmas Island is the world's richest known marine hybrid hotspot: a peer-reviewed study documented 11 hybrid coral-reef-fish pairs across six families (surgeonfishes, triggerfishes, butterflyfishes, wrasses, angelfishes and pufferfishes) where Indian and Pacific Ocean faunas meet — the first recorded suture zone in tropical seas.
• Every wet season (November to March) whale sharks congregate at Christmas Island to feed on the billions of larvae released into the sea during the red crab spawning; juveniles of 3–7 m are most common, and they are most often seen on the sheltered northwest side. Sightings vary year to year and are not guaranteed.
• Each year an estimated 40–100 million red crabs migrate from forest to coast, triggered by the first rainfall of the wet season (usually October–December) and synchronised to spawn before dawn on a receding high tide in the moon's last quarter — and the larvae they release feed the manta rays and whale sharks that visit at the same time.