They followed the trail downwards. In places the slope had been carved into steps as it became too steep to traverse, each foothold sized for a giant. Nina came down a flight, then paused at the bottom. ‘Hold on,’ she whispered.

‘What is it?’ asked Eddie.

‘I can feel a breeze.’

‘All the way down here?’ But he sensed it too, a faint warm draught blowing up the passage.

Cheng looked past the couple. ‘Turn out the torches,’ he said. ‘I think there’s something ahead.’

Barney and Nina did so. As their eyes adjusted, the group saw there was indeed a low light.

‘More glow-worms?’ Nina wondered.

‘If it is, they’re a different colour,’ said Eddie. ‘And they’re flickering.’

‘Disco worms?’ she said with a half-smile. ‘I think we’re almost there.’

They advanced more cautiously. The light grew brighter as they approached a bend, carved steps dropping sharply as the tunnel turned almost back on itself. The breeze became stronger. ‘It’s coming through there,’ Nina said, seeing a ragged opening in the wall.

She climbed onto a ledge to reach the gap. Broken stones were scattered over it; the hole had opened up long after the tunnel’s builders finished their work. More oversized footprints told her the Nephilim had done what she was about to do: look through the cleft to see whatever lay beyond.

The sight took her breath away.

The light’s source was beyond anything she had imagined. She stared at it, then almost unwillingly retreated. ‘You have to see this.’

Eddie took her place. ‘Bloody hell,’ he said, astonished. ‘Dunno what I expected to find down here, but it definitely wasn’t that.’

Beyond the hole was a gigantic shaft within the red sandstone, dropping almost vertically into the unseen depths below. A colossal pillar of crystal over fifty feet thick rose at its centre, disappearing into the red rock ceiling a hundred-plus feet above. The whole thing was aglow in an ever-shifting spectrum of colours, the shimmering light of earth energy coursing slowly through it.

Nina returned to the opening and peered down. The crystal pillar dropped towards infinity below. ‘It must be miles deep.’

Cheng and Barney took their turns to look, both reacting with amazement. The Australian, though, was more than simply awed by the sight. ‘The Rainbow Serpent . . .’ he whispered.

‘What’s that?’ Nina asked.

‘One of our stories – well, the first story. It’s how everything began. The Rainbow Serpent brought life to the whole world from underground. This looks like it, a great snake digging down into the earth.’

She nodded. ‘More truth in ancient legends.’

‘Seems to be a theme with you,’ Eddie noted with amusement.

‘I’ve kinda built a career on it, yes.’ She looked down again. ‘There are some bridges crossing the shaft to the pillar— Whoa!’ She instinctively drew back as she saw movement below. ‘I can see the Nephilim!’