When thunderclouds begin to gather over Botswana’s Kalahari Desert each year, 20,000 zebras get itchy feet. As the first fat raindrops hit the dust, southern Africa’s biggest animal migration gets underway. In a never-ending quest for grass and water, the striped herds undertake an annual epic trek across the vast lunar landscape of the Kalahari’s Makgadikgadi Pans.