Seen from the air, they ripple across the landscape — a river of antelope racing across the vast grasslands of South Sudan in what conservationists say is the world's largest land mammal migration.

The struggle to keep one of the world's largest and rarest antelope species alive has taken a massive step forward in Kenya.

Seen from the air, they ripple across the landscape — a river of antelope racing across the vast grasslands of South Sudan in what conservationists say is the world's largest land mammal migration.