At the crest of Victoria Falls lies Devil's Pool, a natural infinity pool formed on Livingstone Island. While swimming right up to the lip of a massive waterfall seems suicidal, swimmers are protected by a precise confluence of geology and fluid mechanics. The underlying structure is made of resistant basaltic lava layers dating back 180 million years.
Over millennia, the relentless flow of the Zambezi River carved out weaker sediment, leaving behind a resilient, raised lip of hard basalt right at the edge. But rock alone does not prevent a swimmer from being swept away; hydraulic forces play a vital role. During the dry season from September to December, the water flow drops drastically from over 500,000 cubic meters per minute to a fraction of that volume.
As the shallowing river approaches the basalt lip, the rock acts as a submerged broad-crested weir. This obstacle forces the incoming water to decelerate, creating a localized boundary layer where fluid velocity drops significantly right behind the ridge. Additionally, the physics of fluid flow over this barrier generates a localized eddy—a swirling backwater current that actually pushes swimmers back toward the pool rather than over the edge.
Coupled with reduced momentum drag from low river discharge, the friction force provided by the underwater rock wall easily overcomes the remaining hydrodynamic force pushing forward. This delicate physical equilibrium allows thrill-seekers to peer over a 354-foot drop without being dragged down into the abyss.