Maracaibo

The edge of the wall sits about 27 m down, ninety feet by the old chart, and the whole dive is built around reaching it. One operator briefs a free fall of about 100 ft to the edge, and the chart advises a slow descent and keeping your bearings.

The lip you arrive at is a solid crest of coral standing three to four metres proud of the drop-off and leaning out over it in places1. Look over it and the wall falls away under an overhang into blue water, with the coral masses outside running down to 50 or 60 m.

The set piece is the arch. A deep fissure cuts across the lip. You enter it, descend about nine metres and come out on the face of the wall, then swim north to a large arch, perfectly shaped and about nine metres across, at roughly 35 m. You go in from beneath and swim up through it along the fissure that made it. After the arch most guides turn toward shore, which brings the dive to the neighbouring Chunchaka'ab reef. One operator plans about twenty minutes, with the return to the top of the reef, near 24 m, marking the start of the ascent.

The park's own survey used this reef to name a type of reef. The solid, continuous crest that runs along the southern end of Cozumel, in contrast to the separate coral blocks further north, is recorded as locally known as arrecifes de Maracaibo1.

At the rim the reef turns into something you look over. The chart's route description has divers stopping at the edge to see a sheer cliff overhang plunging into deep blue, and the animals people hope for are in the open water off the coral: hammerheads, reef sharks and eagle rays are the names that come up, as possibilities more than promises.

Few divers on the island get to it, because operators gate it. One reserves the right to ask for two or more dives with it first and keeps the site for expert divers, another runs it only from a sign-up list, and one chart grades it Advanced and warns of unpredictable current, large swells and the danger of being lost at sea.

A guide here takes six divers at most. The ascent is an open-water drift off the island's southern end, so a marker buoy is part of the dive, and so is staying level with the group.

Depth is the real limit. Advanced Open Water covers 30 m and the dive starts at 27, so decide the maximum before you descend and keep the turn time in your head. EAN32 reaches its 1.4 PO2 limit at 33.8 m, so ask what the shop can blend and plan to the depth.

Entry into swell is ordinary at the southern tip. The captain makes the call on the morning, so keep a spare day in the week.