Defensores del Chaco is 720,723 hectares — bigger than every other Paraguayan protected area put together, created in 1975 and named for the soldiers of the Chaco War, whose old fortines dot the approaches. It is as close to genuinely empty country as South America has left outside Amazonia.
Cerro León is the landmark: a ring of hills roughly 40 km across rising to 600 m out of dead-flat thorn scrub, visible for an hour before you reach it. Around it is quebracho blanco, palo santo, algarrobo and a great deal of cactus, with rainfall of 500–800 mm and summer temperatures over 42°C.
The wildlife is the point. This is the best jaguar habitat in the country, and the strongest remaining population of the taguá, the Chacoan peccary, which was described from fossils and assumed extinct until live animals turned up in this part of the Chaco in the 1970s. Puma, tapir, giant anteater, giant armadillo and rhea are all here; the frost that can hit on winter nights and the 42°C afternoons happen in the same park.
Do not come casually. There is no fuel, no shop and no reliable water for hundreds of kilometres, the tracks are sand and need a 4WD convoy, and the ranger posts — Fortín Madrejón, Agua Dulce — are the only fixed points. Almost everyone who visits comes with an organised expedition from the Mennonite colonies carrying their own fuel and water. Dry season only, and tell someone where you are going.