Daniel Roux Cave is one of the best known of Christmas Island's many limestone caves, and one of the few that joins land and sea. On land, Upper Daniel Roux Cave is the island's most accessible dry cave, with glistening shawls, stalactites, stalagmites, helictites and pools, and a colony of Christmas Island glossy swiftlets nesting in the upper chamber. Below it, the system continues down to sea level, where a scientific survey of the island's submarine caves (Tan et al., Raffles Bulletin of Zoology, 2014) recorded two openings in the cliff just south of Smith Point: an upper mouth at 2–3 m and a lower mouth at about 7 m, plus an anchialine pool inside with an obvious thermocline and halocline where fresh groundwater floats on seawater.

For divers this is a cavern dive rather than a penetration dive. With a guide, you enter the lower mouth and explore the daylight zone, where the walls are encrusted with sponges and cup corals and the floor is a mix of sand and rubble. Eels are resident – the researchers recorded freshwater eels (Anguilla bicolor) and uniform moray – along with sleeper gobies in the brackish parts, and cave shrimps glint in torchlight. Outside, the coral slope and wall of the north coast offer the usual Christmas Island reef life for the second half of the dive.

The site matters as a window onto the island's hidden karst world: Christmas Island's anchialine caves hold endemic crustaceans found nowhere else, and Daniel Roux is one of the few places the connection between the land caves and the sea can be seen by a recreational diver. It suits divers comfortable with overhead environments who want something different from the walls, and should only be dived with an operator who knows the cave.

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 out of Flying Fish Cove and round Smith Point along the coast for roughly 5 minutes to the cave entrances in the cliff.

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. Surge in the shallow upper entrance makes the cave unsafe with any northerly swell; dive it on flat days only.

Descend on the reef outside, enter through the lower mouth at about 7 m with the guide, torches on, and explore the daylight zone of the cave, keeping the exit in sight. Exit and spend the rest of the dive on the wall and reef outside at 10–20 m, finishing shallow near the upper mouth.

Guided cavern-style dive in the daylight zone with an operator; carry a primary torch and a backup. Going beyond the daylight zone or into the air-filled/anchialine sections needs cave training (e.g. CDAA Cave/Penetration or equivalent), guidelines, redundant lights and gas planning by thirds.

Anchialine pools are scientifically significant (endemic crustaceans) – do not stir sediment or enter without the operator's guidance. Caving (dry) is not permitted inside Christmas Island National Park; check land-access rules for Upper Daniel Roux Cave.

CI Wet'n'Dry Adventures – lists Daniel Roux among its regular sites and guides cavern dives here; Extra Divers Christmas Island – occasional. Dived only guided.