The countryside around Campo Grande is the kind of prosperous, slightly dull agricultural landscape that puts one in mind of Kansas. But just 150 miles to the south near the town of Bonito is a place that more properly belongs in Oz. Water from the Formosa, Securi, and Bama Bonita rivers filters and trickles through a region of calcium-laden bedrock to emerge in pools and natural springs of stunning clarity. Normally, pools of such crystalline clarity offer little in the way of aquatic life, but for reasons unknown, Bonito's pools teem with fish.

The largest of the pools—the Aquario Natural—abounds in dourado, piripitanga, corimba, and hundreds of other colorful tropical fish. Visitors don mask, snorkel, fins, and wetsuit (yes, the water's chilly, but it's that very chill that also keeps out the piranhas) and swim in what is justly called the Natural Aquarium. Equally clear and colorful is a drift along the nearby Rio Sucuri (reached by a one-hour hike) where it's hard to decide whether to look down through the blue-green waters or up at the lush pink and purple stands of orchids lining the riverbank.