The mosaic had been cracked for two thousand years, and Amara was the only one on the trip who had stopped to look at it properly. That was the whole of it, really. She looked, and the crack was wider than a crack had any business being, and then the museum was somewhere behind her and not coming back.

So she did the only useful thing she could think of. She knelt on stone colder than any floor had a right to be and drew the passages in chalk as she found them. Left, left, right, dead end. Her grandmother had told her once that a map is only a promise you make to yourself, and that the difficult part is keeping it.

Three corridors branched away from her now. In the stories she had read a hundred times this was the part where the hero found a thread already waiting, and followed it out, and everyone agreed afterwards that he had been very brave. There was no thread. There was a girl, a stub of chalk, a compass that had stopped pointing north an hour ago, and a sound from the left-hand passage that was not an echo.