The elephants swim across the river in a straight line, trunks jutting out of the water like snorkels. With low, guttural bellows, they push their bodies together, forming a living raft to bolster a calf too tiny to stay afloat on its own.

This pachyderm flotilla has a dangerous destination in mind: The grassy shores of Namibia, where elephants are literally free game for legal hunters. The animals will risk their lives to feed here before fording the Chobe River again, back to the safety of Botswana's Chobe National Park.

To avoid ivory poachers in neighboring Namibia, Zambia, and Angola, elephants like this family are fleeing in astounding numbers to Chobe, where illegal hunting is mostly kept in check. (See National Geographic's elephant pictures.)

But while Chobe offers some protection, it’s not the most welcoming stronghold. The increasingly dry ecosystem is buckling under the pressure of supporting so many of the six-ton animals, which each eat 600 pounds of food daily.

Helicoptering above the park on a searing-hot October afternoon, the landscape looked, as Chase puts it, nuked after a war: Only a few spots of green interrupted a flat, seemingly endless terrain of desiccated trees and brush. (Read more about why northern Botswana is such a hard place to live on Nat Geo WILD.)

Gray hulks, massive even from the air, moved slowly below, following their noses to the few water holes still left at this time of year. The rains should come soon, but in the meantime, the elephants are desperate. The animals can drink river water, but they prefer to drink from remote water holes because rivers are usually risky places to linger.

Once ranging from the coastal plains of Cape Town to the foothills of Mount Kilimanjaro, the species has fallen from 1.3 million in the 1970s to about 352,000 today, according to the survey, which was led by Elephants Without Borders. The International Union for Conservation of Nature lists the African elephant as vulnerable to extinction.

When Chase began collaring and tracking the giants 20 years ago, he was surprised by his initial GPS data, which showed elephants fleeing unsafe territories for safe ones, and then making epic trips back home.

For instance, elephants that had likely escaped Angola during its bloody civil war in the 1970s and '80s traveled hundreds of miles back to Angola in the early 2000s, his data revealed. Other elephants at the turn of the century returned to Namibia and Zambia, where rampant poaching had pushed them out. (Read about the alarming increase in poaching in Zambia.)

Between 2011 and 2014, when the demand for ivory in Asia boomed once again, the elephants' movements stopped dramatically. They stayed put in Botswana, and their numbers are mounting each year. About 130,000 of those survivors now live in Botswana, the most in any country.

In a recent study, Wittemyer and his colleagues found that elephants living in a patchwork of protected and human-dominated land will shift thei

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