Mosi-oa-Tunya – the Smoke that Thunders, also known as the Victoria Falls – straddles the border between Zambia and Zimbabwe. From its broad, smooth-flowing course across a flat basalt plain the Zambezi river suddenly plummets down a fissure in the rock, foaming and churning into the narrow gorge over a hundred metres below. I remember visiting in my childhood: the roar of water, the arc of rainbows in the drenching spray, the smell of wet vegetation, the unprotected edge.