The Piper lowered its bone flute into the silver flame; the yellow-white bone blackened and charred. The second Piper stood behind it, waiting. The Drummers beat out their rhythm. The Dancers swayed.

The Piper straightened up and turned. Ellie breathed out; her head pounded from holding her breath so long. She glanced across at Charlotte, and they crept forward again.

The first Piper knelt, offering the bone-flute in both hands. The second Piper took it, turned towards the wall, then slowly, carefully scored a symbol onto the rock with the blackened end of the flute. Triangular, like a sort of backward-facing capital D: dālet, Madeleine had called it. Door.

The Piper scored another symbol as Ellie and Charlotte crawled through dust and bones. On thy belly shalt thou go, and dust shalt thou eat… Not the best time for Ellie to recollect her long-ago Sunday School classes. The dust caught at the back of her throat. Bones. People. She fought the urge to retch or cough. Beside her, Charlotte let out a muffled sneeze. Ellie glanced sideways; Charlotte’s hand was over her mouth, her eyes wide, but when Ellie looked towards the creatures, they were still engrossed in their ritual.

The Piper completed the second symbol, then began drawing a third. The second mark was the most familiar of all: a vertical line with a diagonal pointing down and to the left. Gīml. Justified repayment; reward and punishment.

They were four or five yards from the fire now; Ellie began fumbling with the clasp securing the backpack’s straps across her chest.

The Piper had finished the third symbol, which Ellie saw with no surprise was hē, the vertical line with three horizontal ones sloping off to the left. Jubilation. The one they’d found where Tony Harper had died; the one that had begun it all.

And would end it, too. The circle was closing. But the backpack’s clasps refused to budge, seemingly determined to stop her doing what she must –

The Piper pressed the charred flute to the cavern wall under the first symbol, scored a long horizontal line across the stone and cast the flute’s remains into the fire. The other Piper began to play again. Ellie felt the tune vibrate inside her, in her guts and bones. The floor began shivering, little ghosts of dust spiralling up from it as thin trails of it spidered down from the ceiling overhead. Another Bible quote came back to her: The earth will shake, rattle and roll.

The back wall of the cavern began to crack along the charcoal line the Piper had scored. But the crack spread out further in both directions, then widened. Chunks of limestone fell away.