Aldabra South Channel

Assumption Island · Aldabra Group · Seychelles

Aldabra is the largest raised coral atoll on Earth and one of the most pristine marine ecosystems remaining anywhere. The north side gets the lion's share of rare expedition visits, but the south face, battered by Indian Ocean swells from Antarctica, is where the wildest diving lies. The reef wall plunges from a shallow coral platform into deep blue water, and channels cut through the rim like notches in a crown, each a highway for pelagic traffic between the open ocean and the vast lagoon inside. We entered at the mouth of the largest southern channel on a flooding tide in April. The current was running at a knot and a half, enough to make finning pointless. We descended fast to twenty metres and tucked against the wall. Gorgonian sea fans the size of dining tables grew horizontally, each a lattice of purple and orange polyps filtering the nutrient-laden flow. Plate corals stacked in shingle formation along every ledge, and massive Porites heads anchored the reef like stone bollards. The first hammerhead appeared at the edge of visibility, unmistakable even as a silhouette, that bizarre cephalofoil swinging side to side at thirty metres. Then three more joined it, and briefly a loose school of perhaps fifteen animals hung in the blue beyond the channel mouth. They stayed for two minutes, circling in a wide gyre, before descending and disappearing. Closer to the wall, a Napoleon wrasse that must have weighed sixty kilograms materialised from a coral overhang and studied us with that peculiar sentient gaze. Grey reef sharks patrolled the channel floor in pairs. A potato grouper the size of a refrigerator sat in a swim-through, its mouth opening and closing in slow rhythm. The drift carried us over a turtle resting station where four green turtles lay wedged into coral alcoves. Above them a reef manta ray cruised the wall edge, cephalic fins unfurled as it funnelled plankton. The scale of life here felt pre-industrial, a glimpse of what tropical reefs looked like before human extraction began.