Thundercliff Cave is Christmas Island's signature dive and the one that most visitors come back talking about. From the reef at the foot of the north coast cliffs, a wide underwater mouth about 10–15 m across at roughly 6–8 m leads into the limestone. Inside there are two caves: a big open chamber lit with deep blue light from the entrance, and a second cave beyond, reached through a smaller tunnel. Divers surface in large air-filled chambers – Extra Divers describe surfacing in a massive chamber open to the sky – where they can take out their regulators, stand on the rock and look at stalactites and stalagmites formed when the cave was above sea level. Diveplanit singles out a very thin cylindrical stalagmite and a 'hanging shroud' formation in the second cave; PADI notes you can surface and continue exploring on foot.

Scientists who surveyed the cave in 2014 recorded two branches: the right ending in a cavern, the left extending about 30 m into extensive air space, with a second anchialine body of brackish water that can be dived to about 10 m. They found rare cave crustaceans including the endemic crab Christmaplax mirabilis. Recreational visitors see cave sweepers, shrimps and, in the dark pools, flashlight fish glowing. Groups stay together and do not venture into the deeper anchialine sections.

Outside, the reef is flatter at about 12 m before dropping steeply, with leatherjackets, ornate butterflyfish, velvet surgeonfish, goldspot seabream, spotted hawkfish, barracuda and occasionally hammerheads out in the blue. The combination of reef, wall and cave in a single dive is what makes it special.

It suits divers with good buoyancy and some experience; operators generally schedule it after a few dives with you. Claustrophobic divers can enjoy the outer chamber, which is spacious and bright.

Fly from Perth (about 4 hours, two to three flights a week, some via Cocos Islands) and hire a car – there is no public transport. Both dive centres are in The Settlement and launch their boats from the jetty/boat ramp at Flying Fish Cove, normally at about 8 am for a two-dive morning trip. From the cove the boat runs west past Smith Point and south-west along the north coast (the 'dog's back') along the coast for roughly about 20 minutes to Thundercliff, below the cliffs.

There is no land access to the dive, and the vertical sea cliffs mean no exit other than the boat. Operators choose sites on the morning according to swell and wind.

On the north coast, which is in the lee of the south-east trade winds from roughly April/May to November – that is the calm, reliable season here with glassy seas and 30 m+ visibility. From about December to March/April the north-west monsoon brings northerly and north-westerly swell (and occasional cyclone-generated swell) that can make the north coast undiveable and the Flying Fish Cove ramp unusable for days; Extra Divers treat February–April as peak swell season when trips are often cancelled. Any swell over about 1.5 m from the north-west quarter usually cancels the site.

Visibility dips after heavy rain run-off from the plateau and during red crab spawning (Oct–Jan) when plankton blooms. Tides are small (about 1 m) and rarely matter, but currents along the coast can pick up without warning. The cave is dived only on flat days, and tide matters: groups plan the visit so the entrance tunnel is not closing off as the tide rises.

Descend to the reef outside at about 12 m, enter the wide mouth at 6–8 m with torches on, follow the guide into the first chamber and surface in the air space. Visit the second chamber via the smaller tunnel, then return the same way, finishing the dive on the reef and wall outside.

Guided cavern dive with an experienced local guide; primary torch plus backup. Penetration beyond the daylight zone or into the anchialine/air-filled sections requires cave training (CDAA Cave/Penetration grade or equivalent), a continuous guideline, redundant lights and thirds gas planning. Air pockets may contain stale air – follow the guide's instructions before removing regulators.

Anchialine waters hold endemic crustaceans – avoid stirring sediment.

Extra Divers Christmas Island – regular guided dive (usually not on your first day); CI Wet'n'Dry Adventures – regular guided dive. Both require torches; dived only in calm conditions at suitable tide.