Jack said, ‘Ningizzida was a very old Assyrian serpent god, traditionally represented as two snakes slithering up a pole, like a caduceus. He was also known as the God of the Tree of Life. Like they did with a lot of other ancient deities, Christianity basically stole him and inserted him into the Garden of Eden as the snake who tempted Eve to eat the apple from the sacred tree. You were pretty young at the time, but we encountered a pit inside the Hanging Gardens of Babylon named after Ningizzida. Had a lot of snakes in it.’

‘The God of the Tree of Life,’ Lily said, looking across at the images above the six tunnel mouths. ‘All six of those images look like the Tree of Life, plus a snake. But look closely and you can see they’re all a tiny bit different. You need serious historical knowledge to get through this place, but it’s also designed to confuse you. Get you to hurry and make a mistake in your haste.’

‘You’re right.’ Jack nodded, seeing it.

‘See, three of them aren’t the Tree of Life at all,’ Lily said. ‘Rather, they’re the Tree of Death.’

‘It’s that one,’ Jack said, pointing at the image second from the right. ‘It’s the Tree of Life—thick lines, ten circles—and it’s got two serpents wrapping themselves up it. Ningizzida.’

Lily gave him a meaningful look. ‘If you’re wrong, and this tunnel—or rather, these tunnels—really are endless, we’re going to go some distance before we realise we’ve gone the wrong way.’

‘Like you said, this is all about knowledge. These traps and choices exist so that only the right people—the worthy—make it to the end of the Labyrinth,’ Jack said. ‘This choice here is the ultimate test of knowledge: are you willing to bet your life, and the lives of your companions, on what you know? I can and I will.’

He pointed again at the door second from the right. ‘It’s that one.’

‘All right, stay here, I’ll go first,’ Lily said.

‘Now, wait a—’

‘No,’ Lily said firmly. ‘You always go first. You gotta let me take the lead sometimes and now is one of those times, since you’re still a little emotional.’

‘All right,’ Jack said.

And so Lily reached up, grabbed the handholds embedded in the ceiling and crossed the dark abyss.

Jack watched tensely as she moved hand-over-hand along the line of ancient rungs.

After a minute of climbing in this manner, Lily came to the other side of the shaft and hung for a moment above the ledge in front of the tunnel mouth Jack had chosen.

For Lily, it was really quite frightening.