We rode westward through the caves, winding our way through a jumbled labyrinth of tunnels. The globe in Nadin’s classroom had shown that Bright Horizon citidome was built in the valley east of the Elysium mountains. I remembered them from when I was little, when we lived in Elysium province, near Lake Amazonis—three giant, extinct shield volcanoes. Well, they were extinct in my time, anyway. Who even knew what they’d be like now? Considering the fact that I was currently riding on the back of a torquing horse-sized Tyrannosaurus Rex, anything seemed possible.

I still couldn’t quite believe what had been through that archway. Even knowing that ancient Mars had been inhabited, I never would have anticipated anything like this. Or that I would get stuck here. It didn’t feel real. I kept expecting to wake up back on Mars, in that cave, with Joseph Condor looming over me and telling me I’d slipped and hit my head or something.

I wasn’t sure what would be worse, honestly.

I leaned forward in my saddle, peering at the tunnel ahead of us. Sunlight streamed in through various holes in the ceiling, casting the sandstone walls in a vibrant orange glow. We were coming up to yet another fork in the passage.

“It’s a good thing the gurzas know where they’re going,” I said, looking over my shoulder at Nadin, “because I sure as heck couldn’t figure it out.”

Nadin didn’t respond. She’d been weirdly quiet ever since we left Eliin’s. Several strands of hair had fallen out of her braid, and she hadn’t even tucked them back in like she was always compulsively doing. As her gurza passed under a shaft of light, I could see that her face was noticeably paler, and a thin layer of sweat beaded on her forehead.

“Nadin? Are you okay?”

She blinked at me, like her eyes were having trouble focusing. “I don’t feel well,” she said.

I frowned, tugging on Tuupa’s reins to slow her down. Thork and Nadin drew up beside us. “What’s wrong?” I asked. “Do you have a fever?”

“I don’t know. The System always regulated my body functions before. Without it, I feel…” She sighed, slumping forward and resting her face against the back of Thork’s scaly neck. “I feel like I’m going to die.”

My pulse jumped. Ordinarily I’d roll my eyes at a dramatic sentence like that, but Nadin looked like she really meant it. “Do we need to go back? Maybe Eliin can help—”

“No,” Nadin protested. “We can’t tell her. She was already suspicious. If she finds out we lied to her…”

“Then maybe we should stop for the day so you can rest.”

“We can’t stop! I have to rescue Ceilos.”

“Yeah, Nadin, but you’re not going to be any help to Ceilos if you’re dead.” The word made me feel nauseous. Taking her earpiece off couldn’t actually kill her, could it? I didn’t believe that could be possible, but she looked so weak. Not like Nadin at all.

“I’m fine,” she insisted. “Let’s keep going. Just a little while longer.”

We rode for another hour or so, until the tunnels opened up into a wider cavern, and the gurzas slowed to a stop. This must be a rest spot of some sort—I could see the blackened remnants of a fire on the cave’s floor, beneath one of the round holes opening up to the surface.

This time, Nadin was too tired to argue with me. She didn’t resist as I helped her off Thork’s back. I braced her weight against my shoulder until she could sit on a large, flat-topped boulder. She drank some water, and when I handed her a blanket from her pack, she bundled herself in it and drifted quickly off to sleep.

The sky through the ceiling holes was turning a deep magenta. It was hard to tell with Iamos’ weird, thin atmosphere, but I guessed it had to be close to sunset. I moved through the large cavern, looking up at intervals to see what I could of the sky above. If I stood at just the right angle, I could just make out the edge of that yellow moon I’d seen my first night here.

Nadin had said it was Venus. But that was impossible. Venus orbited between Mercury and Earth, more than a hundred million kilometers away. On Mars in my time, you could only see Venus at certain times of the year, and it was never closer or brighter than a big star. Jupiter was a lot bigger—sometimes you could even make out the stripes of its atmosphere—but nowhere near the size of this. This was bigger than Phobos and Deimos combined.

I’d thought about it last night, lying awake in the hospital after that riot in the citidome. I vaguely remembered my eighth-grade science teacher saying something about Venus’ poles being upside down, about it rotating backward. Nobody knew why or how, but there was speculation that something had happened to Venus—like a super-huge asteroid collision that had knocked it out of its orbit. What if its orbit in my time wasn’t its original orbit? What if… something had happened to it?

I hadn’t thought anything about it at the time, other than a bland, Huh, that’s interesting. But now it seemed like the worst disaster imaginable. I didn’t know when that impact was slated to happen—but I did know that it would punch a big hole in the geroi’s “evacuate to Hamos” plan. And if the whole population of Iamos had already evacuated to Venus when the impact hit…

I couldn’t tell Nadin. Not after the way she flipped out when she found out I wasn’t actually some far-distant descendant of the Iamoi. And maybe I was wrong. Maybe I’d misunderstood something.

But still. Ceilos’ parents were up there, and who knew how many other people. Yeah, the geroi were creepy, but I didn’t want anyone to die.

I sighed, looking at Nadin curled up asleep on the boulder. Her eyebrows were scrunched up like she was having a nightmare. I should never have made her take her earpiece off.

The cave was growing darker—and colder. Tuupa nudged me, staring unblinkingly with her yellow lizard-eyes. I should probably try to start a fire, I realized. Not that I really had the first idea as to how to go about that, but I wasn’t particularly interested in freezing to death tonight.

I started rummaging around the cave, picking up the most dried-out, woody-looking pieces of spider weed that I could find—and trying to drown out that thought that had been creeping into my mind almost constantly since I’d arrived here.

He’d used the same door as me. But no one I’d met here had ever heard of him. So if my dad had really come here… what had happened to him?

I pushed the thought out of my mind, dumping the kindling onto the burned-out patch on the floor. But as the yellow circle of Venus blinked down at me, I couldn’t help remembering that freezing, airless desert outside the citidome, and the ancient skeleton in the hills.