Nothing but ice and darkness. People say that all Eden was like this once, before the trees came up to the surface: a huge frozen ball of ice. But there was fire deep down in Underworld, the same fire that sometimes comes bursting out from volcanoes. In hot caves deep down near the fire, rock twisted itself into strange new shapes as a lanternflower will writhe and twist if you hold it in a flame. Some of the new shapes that came from that twisting were the first trees. They only grew far underground at the beginning, but then they began to climb up to the surface, melting the ice with their hot hot sap as they reached out into the air. The lowest parts of Eden’s surface filled with water and became Worldpool, and the trees that grew there became the shining watertrees that I used to see beneath me when we went out on the bright water from Knee Tree Grounds, their soft branches waving back and forth. Middle parts of Eden were opened up to the air as the ice melted and flowed down to the Pool, and so the trees there became forest, lit up by their lanterns, and filled with the constant sound of the muscles pumping inside their trunks, bringing up the heat that kept the ice at bay. But in the highest parts of Eden, the parts that were closest to the cold black sky, the ice never melted, and light never came.

We trudged through the snow, warmed ourselves beside the little fires at the guard camps, and trudged on again in the little pool of light from the headlantern of our only woollybuck. Outside that pool was darkness, but in the distance there were other little patches of light, ahead of us and behind us, with little shadowy groups of human figures inside them. Like a necklace of glowing white beads laid out over the dark ground, these patches of light wound their way along the side of a mountain and up to a crossing place ahead of us where there glowed a single red bead of fire.

I walked up beside Ugly to check the two little kids were okay, and found both of them asleep in their father’s arms, far away from all of this, at least for a little while. Dave himself stared out miserably over their heads at the mud and the snow and the darkness. Fox was walking up in front beside the woollybuck with his uncle Tom, who sometimes liked to tell him stories about his time in the guards.

‘I’m really not sure this was a good idea,’ Dave muttered. ‘We’re already cold cold and there’s a long way to go. Plus, I don’t care what you and Tom said about snowleopards, they’re still up here. What chance would we have against them if one came?’