The Buna is nine kilometres long. It arrives full-size out of the base of a cliff, runs for an afternoon, and is finished. This is the last 850 metres of it.

Almost everyone who comes to Herzegovina sees the start. Vrelo Bune, at Blagaj outside Mostar, is one of Europe’s great karst springs — a river emerging at full volume from under a cliff face, with a centuries-old dervish house built at the water’s edge. It is on every itinerary, it carries an IUGS geoheritage designation, and it deserves both.

Then the Buna runs west through Kosor, Malo Polje and Hodbina, and nine kilometres after it began it is out of river. The confluence with the Neretva is where this find is: a stretch known as the Buna Canals, about 850 metres of it, where the water is forced through a limestone channel barely wider than itself before the larger river takes it.

The colour is the reason to stop. Karst water comes out of the rock cold and carrying almost no sediment, which is why the Buna is that particular hard turquoise rather than the green-grey of a lowland river. Running it through a narrow slot at speed makes the point twice over: white where it breaks on the lip, and clear enough down the middle to see the channel floor.

The setting does the rest. Willow and scrub crowd one bank; the other is bare terraced limestone stepped down like a quarry floor, with side-falls spilling over the edge into the main flow. Karst hills close the valley ahead. There is no ticket office and no viewing platform, which is the character of the place — you park, you walk to the edge, and that is the visit.

The honest note is that this is a stop, not a day. You are looking at moving water in a rock channel for as long as moving water in a rock channel holds you. It sits a few minutes from Mostar, between Blagaj and the Old Bridge, which is exactly why so few people stop: it is in the gap between two things everyone has already decided to see.