Lost Lake Cave lies on the outer edge of what Extra Divers call one of the largest cave networks on the island. Christmas Island is a raised coral limestone island, and its cliffs are riddled with solution caves; at Lost Lake the system reaches the sea along the base of the north coast cliffs. The 2014 scientific survey of the island's submarine caves (Tan et al.) described Lost Lake as one of the most extensive explorable marine cave systems on the island, highly dependent on tide and wave conditions, with inland anchialine sections that can otherwise only be reached after a walk of three to four kilometres.

The recreational dive is on the outside and in the entrance zone. Extra Divers give a depth range of 11–25 m and describe a unique topography with something new around every corner – undercuts, overhangs, gullies and cave mouths set into the reef and wall. Light streams into the entrances, silhouetting divers against the blue behind, and the shaded ceilings carry sponges, cup corals and sea fans. Outside, the wall and reef hold the usual north-coast life – fusiliers, butterflyfish, triggerfish, groupers and reef sharks – and Extra Divers note whale sharks in season.

The site suits intermediate divers who are comfortable near overhead environments and want more than a straight wall dive. The deep interior of the Lost Lake system is a serious cave and is the preserve of properly trained and equipped cave divers and researchers; recreational groups stay in the daylight zone with a guide.

Lost Lake East, a reef and wall dive nearby, is often dived as a separate site. Together with Thundercliff and Thunderdome to the west, this stretch of coast is the heart of Christmas Island's cave diving.

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 Lost Lake.

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 entrances are highly tide- and swell-dependent (surge funnels into them), so the cave section is only dived in flat conditions.

Descend on the reef outside at 11–15 m, follow the guide along the base of the cliff and into the entrance areas, staying within sight of daylight, then continue along the wall at 15–25 m before returning to shallower reef for the safety stop.

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 – lists Lost Lake Cave among its sites, dived guided in calm conditions; CI Wet'n'Dry Adventures – dives the Lost Lake area (see Lost Lake East).