Elliot threw the cube at the boulder, and the sudden movement made Harri flinch. It struck the huge rock and shattered, and shards of crystal glinted as they flew everywhere. The glass ball broke next, smashing into a multitude of tiny pieces. The stars inside scattered over the rock, but rather than simply glitter on the craggy surface, the most marvellous thing happened.

The stars unfolded to create tiny hexagons of energy, and the hexes multiplied, spreading across the surface of the rock to form a mesh of shining geometric shapes. When the mesh was complete and the rock was encased, there was a flash of energy and the rock and shimmering hexagons vanished to reveal a large cave mouth.

Harri looked at Elliot in astonishment. If he shared her disbelief, he didn’t show it. Instead, he seemed at peace. Were the stars some kind of caustic or corrosive substance? Was that how Ben and David had disposed of Beth’s body?

Harri was startled when Elliot grabbed her arm and pulled her forward into the yawning cave mouth.

‘Come on,’ he said. ‘We don’t have long.’

She allowed herself to be led into the darkness.

‘Move,’ he urged, pulling her further inside.

Harri couldn’t understand why he was so concerned, but then to her dismay there was a thunderous crack and a great tremor, as though reality was being rent asunder, and the huge boulder materialized, covering the cave mouth and plunging them into total darkness.

Chapter 42

Harri was stunned. She could hear Elliot beside her, but she couldn’t discern even the faintest shape.

‘What just happened?’ she finally managed.

‘I’m not sure.’

‘I’ve got a torch on my phone.’

‘There’s no need,’ Elliot told her.

Harri heard him moving in the darkness, and soon afterwards there was the click and thud of a large switch locking into place, and two lines of LED lights came on, illuminating a long tunnel that ran deep beneath the cliff. The lights reminded Harri of a runway, and she couldn’t shake the feeling they would take her somewhere terrible.

‘What is this place?’ she asked.

‘Ben brought me here the night DS Khan died,’ Elliot replied. ‘I was so upset I wasn’t sure what was real and what wasn’t, but he said he wanted me to see it, that it was important I knew it existed. I came here once, about a year after I’d moved in with Mrs Hughes. I wanted to see if it was real, but I couldn’t get past the rock. For years, I thought I dreamed this place, but it seems not.’

Elliot started down the tunnel, and Harri followed. The LED lights were attached to bare stone that had been worn smooth, perhaps by an underground river. They were connected to each other by thick cabling, and at an interval of every five fittings the cable ran into a glowing cylinder about six inches long and one in diameter. The cylinders were filled with what looked like the same stars that had consumed the boulder. Harri touched the nearest cylinder and felt a gentle warmth.