Una

Una is the Federation's only national park and the country's most purely beautiful one: the canyon of the upper Una from Lohovo upstream, the lower gorge of its tributary the Unac, and the karst plateau between them, declared in 2008 along the river that gives the park — and half the border with Croatia — its line. The water is the whole argument. The Una runs an improbable emerald-turquoise over beds of sedra, the living tufa that calcifying moss and algae build into barriers, and every few kilometres the river steps down one of them in a sheet of white. The youngest tufa near the springs is around three thousand years old; the oldest formations have been growing, dam by porous dam, for perhaps three hundred thousand.

The two great set-pieces bracket the park. Štrbački buk, the Una's highest fall, drops 24.5 metres over three broad terraces where the river briefly doubles as the state border — Croatia owns the far bank — and is the image on every Bosnian tourism poster that doesn't show a bridge. Upstream at Martin Brod, where the Unac meets the Una, the river dissolves into the park's most intricate water: an 800-metre labyrinth of cascades and channels descending 54 metres through the village, known collectively as Milančev buk and listed on UNESCO's tentative list as the finest travertine complex in the country.

People have crowded these fords since antiquity — the Japodes before Rome, then a Roman crossing whose legionaries, the story goes, called the river simply una, "the one." Kulen Vakuf, the largest settlement inside the park, keeps the shape of its Ottoman market days, with the ruined Ostrovica fortress on the crag above and Havala's walls across the valley; at Martin Brod the little Orthodox monastery of Rmanj, medieval in origin and burned and rebuilt more than once, sits a short walk from the cascades. Otters and trout own the river; wolves and bears keep to the Plješevica and Osječenica forests on the rims.

What's worth knowing before you go

Bihać, half an hour north, is the base, with the park office and most outfitters; entry to Štrbački buk (via Gorjevac or the Orašac side) and to Martin Brod is ticketed at modest fees. The park's classic activity is rafting — the Štrbački buk to Lohovo run mixes rapids with long emerald pools — and late July brings the international Una Regatta, a days-long flotilla down the river. Boardwalks at both waterfalls make the set-pieces easy; swimming is best in the sanctioned stretches near Kulen Vakuf.

Water is the timing question: spring snowmelt makes the falls thunder but muddies nothing — the Una clarifies almost immediately, which is part of its magic — while late summer turns the terraces gentle and swimmable. The connoisseur's call is Martin Brod over Štrbački buk: a fraction of the visitors, cascades you wander through rather than view from a platform, and Rmanj's quiet cemetery above the water.