Mucum beamed, full of himself as he plonked his helmet down. “That was … out of this wood! You know, I’d swap jobs with them lot any day.”

Ark shook his head, incredulous.

“Glad you’re having so much fun.”

“If you’ve got any memory left in that bonce of yours, you might recall it was your idea to climb down that Diana-forsaken ladder in the first place.”

“We had no choice. Anyhow, we’re not exactly saving the country down here!”

“You sound like a moaning grandma sometimes. Lighten up!”

“Speakin’ o’ light,” interrupted Joe, “these are fer yow.” Joe handed each of the boys a cloudy glass bottle. “Shake ’em hard, but only if yow need to. We’ve enough to see by fer now.” Joe set off up the phosphor-coated passage. “Follow me close and stay in the center.”

The squidgy silt reminded Ark of … what was it? Oh yeah … the stuff they worked with every day. He sniffed. The smell was different, loamy. It made him think of fallen leaves and last year’s mulch pile in the corner of the scaffields. They were a long way from the sunshine now. His eyes picked up the increasing glitter of the roof and floor. Copper? Tin? Iron? Ark had no idea.

Joe had explained about the natural seams that the roots of every tree sought out. All the miners had to do was follow.

“But this is just one root system?”

“Yas!”

Mucum’s mind began to boggle, trying to work it out. “And there’s like … thousands of trees!”

“Yas!”

“So, what? There are others like you?” The thought of endless women Rootshooters fluttering their eyes at him gave Mucum the horrors.

“Yas!” A slow smile spread across Joe’s face. “This ’ere’s just one way in the many. Them roots is tangled together like breathin’ tubes, as if the trees war all one big family! If yow wants, you could cross the ‘ole country without ever comin’ up into a divin’ station, let alone goin’ up top! Why, Oi’ve got kin out west and south, all workin’ diff’rent systems!”

As above, so below, thought Ark. Arborium had just got a lot bigger.

They continued down the main root. The plants overhead dripped with life, shocks of vibrant green in the fungal illumination. Every so often, side passages led off both left and right, bringing with them a cold chill that sent goose bumps marching up and down the boys’ skin.

“Don’t move,” hissed Joe suddenly, his frame coming to a standstill.

The boys almost collided with him. What now?

“Up ahead …,” Joe whispered. “Slow now. Back up, quiet loike. I don’t think we’ll be mining any ore today….”