“You can take off your breathing apparatuses now,” said the woman the others had called Marin. “We don’t need them from this point on.”

I stared upward, trying to find the source of the light, but the cavern’s ceiling was so far above us I could barely see it. I was so focused on what was above that I didn’t even notice the floor until I stepped on something soft and gasped. Under my feet, the stone was covered over with a carpet of green plant life. It was thick, crawling over boulders and stalagmites, climbing up the wall in places. Here and there, patches of ferns emerged from cracks in the floor and walls, covered in new, spiraling branches that curled and unfurled like a slithering anguis. Further down, a narrow channel coursed alongside the path, and thick, leafy bushes sprouted from its banks. The slow-moving stream burbled pleasantly as we passed it.

Everything was green. The black, gnarled branches that I’d become accustomed to in the citidomes were nowhere to be seen. It looked like images I’d seen on the visual indicator of Iamos decades ago, before the climate disaster.

“What is this place?” I breathed.

The man holding my arm smirked. “Elytherios.”

The tunnel was widening, growing brighter and brighter. Ahead of us, I could hear the rushing of water. Then the high ceiling disappeared entirely behind a patch of white mist, and a whole world opened out before me.

The cavern here was so massive that you could no longer see the walls of it. I wondered for a moment whether we were even inside anymore, but nowhere on Iamos’ surface looked like this. Thick clumps of trees stretched skyward, their branches—laden with new growth and small, unripe fruit—reaching toward the unknown light source. Beyond the trees, untidy rows of crops filled the landscape, tall green stalks around which climbed snaking vines. Between the rows of crops, makeshift waterspouts splashed noisily. In the distance, I could just make out the roofs of small buildings. And above our heads, wispy white clouds swirled on the hint of a breeze.

My knees wobbled, threatening to give out entirely. Only the man’s strong grip on my arm kept me upright.

“Is this the inside of the volcano?” Isaak said behind me.

Marin tugged her earlobe. “It’s a hollow dome, left from the last eruption. The magma chamber collapsed beneath it, just leaving the outer shell.”

“And it formed a microclimate?”

The other woman chuckled. “Well, it had a little help. Elytherios’ engineers are the best on Iamos.”

Isaak’s eyes widened. “You terraformed it?”

The anarchists looked at each other, murmured a few words I couldn’t quite catch. Then they prodded us forward again.