He’d expected to remain in blackness, but the tube around him now glowed softly green, with brighter sparks flashing as he whistled past. Joe had mentioned a phosphorescent fungus that grew lower down along the walls of these massive, hollow tree roots. Ark was grateful even for this echo of the far sun. The fungus spread above him in yellow lines scribbling away at the shadows. Who would have thought lightning could be grown?

“This is serious!” whooped Mucum, as he zipped down the smooth-worn slope. He quickly worked out that his suit was engineered with this exact journey in mind. The trick was to go with it, bending your body into the curves, hitting the turns, and using your butt to bounce off and accelerate out and down.

There was a loud splash and all three of them were suddenly underwater.

Ark was last to crash into the gleaming pool. Every nerve in his body went taut and his lungs worked overtime, grabbing at the air. He was drowning, imprisoned in some contraption dreamed up by a species of underworld nutter. The visor in his helmet began to steam up as he hyperventilated.

“Help!” he screamed, the sound swallowed inside his suit.

Joe floated up to him and grabbed Ark by the shoulders. He moved his right hand up and down in the water in slow motion, his goggled eyes willing Ark to look.

Ark got the message. He tried to slow his breathing. Trust that the insect boatmen were working the bellows at the other end. Slowly, the mist cleared from his visor. Ark moved his hands forward experimentally. The rubber suit acted like warm grease, slipping him easily through the chilly depths. It was like being in an enormous cruck pool.

He realized this was the water table, where the tree drank its fill and nature pumped the liquid of life high into the estate. As he began to relax, Ark watched a shoal of shimmering red ovalfish glide by, thin as paper, with eyes on either side of their heads.

Joe beckoned the two boys to follow and swam toward what appeared to be a tunnel opening. They clambered through into a shallower pool, half filled with silt.

Above the trio’s heads, a million fronds hung from the roof like frail curtains. They were way below the earth by now, a place of far tales. An occasional glint winked back at them from the roof of the giant root — evidence of ore, according to Joe.

There was a hiss of escaping air as Joe undid the seal on his helmet. Ark copied the Rootshooter’s movements, glad to be able to breathe normally again. He heard the churning ripples of deep water and the constant plop of the dripping walls.

“We should be safe fer about an hour. Leave yow helmets and breathin’ lines behind and follow me. Yas?” Joe was all business now, in his element.