Iguaçu Falls — Cataratas do Iguaçu in Portuguese, Cataratas del Iguazú in Spanish — are one of the great natural spectacles on earth. The Iguaçu River drops over a basalt cliff edge in a horseshoe nearly three kilometres wide, splitting into roughly 275 individual falls that thunder into the gorge below. Rainbows hang permanently in the mist, and the roar carries for miles.
The falls straddle the border between Brazil and Argentina, and both sides are worth visiting. The Brazilian side offers the panoramic view — you can see the entire curtain of water in a single sweep from the walkways that jut out over the gorge, culminating at the Garganta do Diabo (Devil's Throat). The Argentine side lets you get closer, with catwalks running right over the top of the falls and trails through the surrounding Atlantic rainforest.
The base for visiting is Foz do Iguaçu on the Brazilian side or Puerto Iguazú on the Argentine side. The Itaipu Dam, a massive Brazilian-Paraguayan hydroelectric project on the nearby Paraná River, can be visited on a free guided tour.