Perpendicular Wall is Christmas Island's most celebrated boat dive, and Extra Divers rate it one of Australia's most dramatic wall dives: a vertical cliff at the north-western tip of the island that falls away into the abyss. The name is literal – the face drops almost vertically from about 5 m, and the island's volcanic flanks continue down for thousands of metres.

Descriptions from Diveplanit, Divernet and Scuba Diver Life agree on the character. The dive often starts at an overhang or shallow cave with red sea fans, then continues along the wall, where corals and gorgonians lean outward to catch light and current. Sea fans, feather stars and hydrocorals make the face a riot of colour. The fish life moves in two directions: hundreds of black triggerfish and pyramid butterflyfish and schools of fusiliers streaming up and down the face, reef fish moving along it, and blue-lined angelfish among the invertebrates. Sand cascading down from above settles like snow on the coral. At around 32 m a school of batfish is often seen off the wall.

Out in the blue, this is Christmas Island's big-animal site. Extra Divers list sharks, sailfish and whale sharks; Divernet mentions a strong possibility of manta rays and whale sharks; and dolphins are frequently seen from the boat before the dive. The return is along a shallow reef at about 5 m, ideal for a long safety stop.

The site is advanced because of the combination of a bottomless wall and currents that can be strong, and because the long boat run means it is only possible in calm conditions. It suits experienced divers with good buoyancy and depth discipline. Those who dive it tend to agree with the Diveplanit author's advice: slow down, because there is plenty to see.

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 along the full length of the north coast to the north-west corner of the island along the coast for roughly 40–45 minutes to Perpendicular Wall.

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 exposed west coast, a long run from Flying Fish Cove, so it is only dived on calm days. The site is sheltered from the south-east trades (roughly May–November) but wide open to north-westerly monsoon swell (December–April) and to long-period south-west groundswell generated in the Southern Ocean, which wraps around Egeria Point. Operators generally need less than about 1–1.5 m of swell and light winds to make the trip; the calmest spell is usually August–November.

The west coast has a reputation for stronger and less predictable currents than the north, and for the clearest water on the island (often 40 m+). Visibility can drop during red crab spawning (Oct–Jan). Cyclone season is November–April.

Currents at the point can be strong; operators time the dive for manageable flow.

Descend at the overhang with its sea fans near the top of the wall, then drop to 20–30 m and follow the wall with the current, keeping it on one shoulder and watching the blue. Around 32 m look for the resident batfish school. Ascend gradually along the face and finish on the shallow 5 m reef on the return, with a DSMB for pick-up.

Nitrox (EAN28–32) helps on repetitive deep wall dives; deep specialty recommended for the 30 m+ batfish zone.

Extra Divers Christmas Island – regular boat dive in calm conditions (about 45 minutes each way); CI Wet'n'Dry Adventures – regular, depth-limited to 25 m. Usually combined with North West Point or Boat Cave as the second dive.