The monasteries of Metéora are indisputably one of the great sights of mainland Greece. These extraordinary buildings, perched on seemingly inaccessible pinnacles of rock, occupy a valley just to the north of Kalambáka ; the name metéora means literally "rocks in the air", and kalabak is an Ottoman Turkish word meaning roughly the same thing. Arriving at the town, your eye is drawn inexorably towards the outermost of these weird grey cylinders. Overhead to the right you can make out the closest of the monasteries, Ayíou Stefánou, firmly ensconced on a massive pedestal; beyond stretches a chaos of pinnacles, cones and stubbier, rounded cliffs. These are remnants of river sediment which flowed into a prehistoric sea that covered the plain of Thessaly around twenty-five million years ago, subsequently moulded into bizarre shapes by the combined action of fissuring from tectonic-plate pressures and erosion by the infant Piniós River.