Thunderdome sits right beside Thundercliff Cave on the north coast, but it is a very different dive. Its entrance is hidden behind a large rock and is entered through a split, opening into a dome-shaped cave with a sandy bottom and walls encrusted with colourful growth – sponges, cup corals and encrusting invertebrates (Diveplanit, Divernet). The Tourism Association lists it among the island's featured dives.

The 2014 scientific survey (Tan et al.) recorded a wide mouth about 10–15 m across at around 17 m depth, a shallow extension to the right, and a left passage of 20–30 m leading into an extensive system that opens into an air pocket over an anchialine pool. The researchers found rare crustaceans here, including the cave crab Neoliomera cerasinus, the furry lobster Palinurellus wieneckii and the reef lobster Enoplometopus voigtmanni.

Divers who have been further in describe a genuine cave dive: beyond the entrance there is no daylight, the passages twist and branch ('after the first seven or eight turns I was lost', wrote a Perth Scuba trip leader), and in places the group can surface in air pockets to talk and look at formations built by millions of years of dripping water. A sinkhole inside holds small crabs at the bottom.

That is why Thunderdome is graded here as a cave dive. The dome and entrance zone can be visited as a cavern dive with a guide, but the inner system is an overhead, zero-visibility-if-silted environment where navigation depends entirely on a guide and a line. It suits experienced, cave-trained divers or small groups led by the local guides who know it intimately. For most visitors, Thundercliff next door gives the air-chamber experience with far less commitment.

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 Thunderdome, next to Thundercliff Cave.

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. Dived only in flat conditions; any swell creates surge at the entrance and stirs silt inside.

Descend to the entrance rock, pass through the split into the dome with torches on, and explore the dome's sandy floor and encrusted walls. Cavern-level divers turn back within sight of the entrance; cave-trained divers following the guide's line continue into the left passage to the air pockets, then return the same way.

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. Beyond the dome: continuous guideline mandatory, three light sources, thirds rule, sidemount or well-trimmed backmount.

CI Wet'n'Dry Adventures – lists Thunderdome among its regular sites and guides small groups into the system; Extra Divers Christmas Island – occasional. Only ever dived with an experienced local guide.