Travertine terraces are normally white. The hundreds of shallow pools stepping down a mountainside at Badab-e Surt, in the Alborz of northern Iran, are cut into rust-red stone. The difference is one extra element: two mineral springs rise on this slope, and one of them carries iron along with its calcium. As the water surfaces and loses its carbon dioxide it drops both, and the iron then oxidises, staining copper, ochre and crimson what would otherwise have been as white as Pamukkale. The springs have not finished. They are still laying stone, and each rim is fretted with fresh crust: thin ledges and frozen ripples of carbonate tinted rust and cream. A boot breaks in a second what the water takes centuries to build, so you keep to the marked edges. The pools run warm enough to steam on a cold morning.
The slope is worth seeing twice over. From the top edge the basins line up as one continuous flight spilling into each other, hundreds of them in orange and red down the hillside, and only from up there do you get the scale. Then, low on the slope on a windless day, you crouch to water level at a full basin and the coloured steps and wooded hills appear twice, once in stone and once in the surface. The hour decides the rest. Before the sun clears the ridge the whole hillside sits in a pre-dawn grey. As the light climbs it drags the reds and oranges up out of it, one step at a time, and the mineral colours run deeper than anything you see at midday. The second spring never became a sight at all. It runs so heavily salted that people have come here for generations not to look but to sit in the small warm pool it feeds, for aching joints and bad skin, a working spring set apart from the colour below it.