Martvili Canyon is a limestone slot in western Georgia, in places barely five metres from wall to wall, with the Abasha river running emerald along the bottom of it. The stone arches crossing overhead were never built. This rock was riddled with caves before the river found its way through, and where the roofs gave way the passage was left open to the sky; where a span held, it now bridges the canyon. So the gorge is really a cave with most of its ceiling missing. The walls it left are a Cretaceous seabed, and along the water-polished rock you can pick out the shell profiles of ammonites and sea urchins. The same beds gave up the footprints of plant-eating dinosaurs to an expedition in 2010. Closing over all of it is Colchic forest, yew and beech and chestnut with an evergreen understorey kept lush by air off the Black Sea, a relict of the woodland the ice ages scoured off the rest of Europe.
A paved trail of 700 metres runs the rim. Side streams arrive over the lip in falls of twelve to fifteen metres and land in a pool the colour of bottle glass. On the platform beside them the mist finds you, and the ledges stay green and dripping through the driest month of the year. Then the path hands you thirty limestone steps, cut for a princely family who swam here before 1867, down to Dadiani's Bath, the deepest and most sheltered water in the gorge. Nobody gets into it now; the pools here are for looking at. The last 300 metres are the part no trail reaches. You board a raft at the still pool where the walkway ends, and an oarsman takes it into the slot. The colour in there depends entirely on the river: deepest emerald in August, once the sediment has settled, and high and cloudy through the spring melt, when the rim falls thunder and the boats stay ashore. Overhead, as you go under it, is a piece of cave roof that never came down.