And if that is not enough to put off the hardiest of adventurers, the Chaco forest, which is twice the size of the UK, is known as “green hell”. Exotic wildlife includes jaguars, pumas, giant anteaters and otters, prehistoric lungfish and the great fighting river fish. In summer it is a dustbowl; in winter the rains come and turn it to glue. Although the forest covers almost two-thirds of the country, only four per cent of the population lives here. “It’s fanciful to say that the tribes who have survived there have done so because of their spears,” says Gimlette. “They’ve survived because no one else wants to live there.”