The chamber we are in is enormous, the vaulted roof as high as that of a church and at least 30 meters from wall to wall. On the sides, gentle slopes of bare rock come down towards the water, forming a sort of outcrop where there is a bend in the river, the boat skimming across the gravel just inches below the surface. The whole scene illuminated by the greyish light of our flashlights as they focus on different parts of the surroundings.