The word stuck in Ark’s brain. Diving? He always enjoyed his swims in the remote cruck pools hidden high in the canopy — there was a perfect spot where the water was cool and the fish ticklish on the toes. He often liked to venture there on his own, to spy out the darting kingfishers and share his lunch with the squirrels. But those days, filled with lazy sunshine filtering through green leaves, were long gone. He had a feeling that this was diving of a different sort.

Mucum had gone quiet, and Ark looked around him in awe. The station was a huge hollow cavern at least half a mile across, sides sloping in toward the roof way above. They must be near the bottom of the trunk, deep in the heart of the tree. Light was provided by various pools of what looked like water but had to be liquid gas. Flames danced and twisted on the shimmering surface, filling the cavern with an acrid smell. And throughout, a tangle of enormous tubes twisted out of the cavern floor and made its way up to the roof and beyond. A forest inside a tree.

Ark stared at the liquid, mesmerized. “We … we’re not going to swim in those, are we?”

“Ah, no,” chuckled Joe. “That be liquid gas! We be goin’ down them pipes. “We call ’em Xylem. Oi found yow in one of ’em.” He pointed at the boys, then back at the tubes as if Ark and Mucum were simply another product of mother nature. “Theys are part of ’Er Maj’s plumbin’ system! Water ‘n’ gas! The stuff of life! Yow lot up top have it easy. Weren’t for us, yow wouldn’t be livin’ so snug!”

Mucum still stared. This was an unwoodly place. On the far side of the hall, partially obscured by the forest of Xylem and the single huge column of heartwood that supported the roof and the tree above, they could see and hear rumbling conveyer belts.

Joe explained the process that began with his fellow mineral miners reaching deep beneath the earth, led by the living tree roots to excavate rich seams of copper, lead, and iron-bearing lode. Once transported up into the station, the ores were separated out, using giant magnets, before their long trip by dumbwaiter and barge to the Blacksmith estates down south.

“Isn’t the earth” — Ark almost whispered the word—”only on the other side of these walls?” His eyes scanned the cavern without seeing any doors. In his mind, he pictured a foggy mud-swamp punctuated by the rearing arrows of smooth-skinned trunks.