Malpelo

There is no infrastructure out here. A small Colombian navy detachment holds the rock, divers do not land, and the liveaboard is the only shelter for hundreds of kilometres, with no harbour to run to if the sea gets up. Diving works off tenders that drop you beside a wall and collect you in swell afterwards, so surface markers and the habit of coming up as a group matter more than they do on a house reef. Only a small number of vessels hold sanctuary permits, and their dates go far ahead.

The island is a single cliff-sided monolith with satellite islets scattered around it, and it behaves the same way underwater. The walls fall away steeply from close to the surface with almost no shallow ground to potter on, so nearly every dive is a wall or a detached pinnacle hanging in blue water. Cracks and overhangs give the fish somewhere out of the flow while the exposed corners collect the current. Above the waterline the cliffs are white with seabird colonies that feed on the same food chain the diving depends on.

Exposure protection is the thing people get wrong. The latitude suggests a thin suit, and then a dive spent hanging in the cold layer — often where the shark activity concentrates — turns into an endurance exercise; hoods and 5 mm or thicker suits are the norm. Current is the other variable, changing strength and direction between dives on the same site, so the shape of the dive tends to be decided by the guide in the water rather than agreed on deck beforehand.

Malpelo rewards a narrow kind of diver. Coral cover is thin, there is no macro programme to speak of and no sheltered shallow site to fill an afternoon, so a poor-visibility day leaves nothing to fall back on. It tends to be a second or third Eastern Pacific trip rather than a first, chosen by people who already know what a hammerhead school looks like and want the rawest version of it. The smalltooth sand tiger sightings that made the rock interesting to researchers are reported mainly from depths below the recreational range.