The hidden journey made by 20,000 zebras

Botswana's Nxai Pan National Park -- part of our planet's biggest expanse of salt pans -- is so isolated that the world's longest annual mammal migration, which crosses these pans, was not even documented until a decade ago.

(CNN) — The chopper rose smoothly from Maun Airport and we hurtled ahead over Thamalakane River. I knew it was a 'river' only because I could see a bridge -- there was nothing about the sandy ribbon of dust to suggest it had ever been touched by water.

It was hard to imagine that, just 30 kilometers (18.6 miles) away, the world's largest inland delta sprawled northwards, stretching beyond Namibia's Caprivi Strip border.

This is Botswana's Nxai Pan National Park -- part of our planet's biggest expanse of salt pans. It's an isolated region where the 600 square kilometers of Okavango Delta finally seeps away into the insatiable sands of the Kalahari Desert -- aptly known to early travelers as "the great thirstland."