Wadi Shab is a gorge in Oman where a chain of spring-fed pools runs between limestone cliffs and ends at a crack barely wider than a head. It cuts down out of the Hajar Mountains to the coast beside the village of Tiwi, and its water is there all year. Rain soaking into the mountains travels underground through the limestone and seeps back out along the wadi floor, so the pools keep running through summers with no rain at all. The walk in takes about forty-five minutes and has no shade on it. Stone terraces start soon after the trailhead, stacked in narrow steps with a cut channel along the top of each — the falaj that watered them by gravity, as Omani villages have farmed for over two thousand years. Date palms, bananas and limes still fruit on plots nobody tends. The boulders grow as the walls draw in, and where the trees lean over the water the sun goes off you and you can hear the stone running.
Then the pools begin, clear green and cold and deep enough to swim across, one after another up the gorge. The last stretch cannot be walked at all. You leave your bag on the rocks or seal it into a dry one, swim between narrowing walls until they almost meet, and push your head through the crack against the water coming out. Behind it is a half-flooded chamber, dissolved out of the limestone like the rest of the gorge. A waterfall comes straight down the rock, with a rope beside it for anyone who wants to climb and jump back in. Rain anywhere upstream turns the gorge into something else: the walls are sheer, and a flood has nowhere to go but along the floor you walked in on. On a dry day there is only this: nothing to hold on to and nowhere to stand, so you tread water while the fall lands a few metres from your face.