Truso is a flat-bottomed valley high in the Georgian Caucasus near Kazbegi, where the Terek is born, and the ground the river starts on is stained rust-orange by springs that run cold. Terraces like these are normally the work of hot water. Here the rain sinks into the limestone, takes on carbon dioxide and dissolved minerals far below, and comes back up cool. The gas escapes at the surface, the water can no longer hold what it is carrying, and carbonate and iron settle out in sheet after sheet. The iron does the colouring, ochre through yellow to deep red, across whole stretches of the valley floor. Beside the sheets lies Abano lake, churning and bubbling as though it sat on a fire, and cold when you put a hand in. That fizz is carbon dioxide as well, and the air above it turns sharp and eggy.
The walk keeps to the floor beside the young Terek, which comes down the middle in shallow braided channels with meadows on both sides and snow-veined peaks closing the view ahead. It is easy, and long enough to take most of a day. That is how you get the valley in order: travertine fields, then the emptied villages, then the towers. You can also hire a local horse and ride out the way the herders still do, splashing through the braids. Ketrisi and the other hamlets are half gone — roofless stone houses and old byres among the few homes still lived in. A census in 1939 counted more than three thousand Ossetians across these villages; war in 1991 and a border that hardened after 2008 emptied them, and some two dozen people remain. Medieval defence towers still stand over the gorge, and a short steep climb reaches the ruined fortress of Zakagori on its ridge. From up there the rest of the valley is as far as you get: the Georgian border post at the fortress turns walkers back.