I nearly dropped the jacket into the puddle, but caught it just in time. This was my dad’s work jacket. He’d been wearing it the last time I’d seen him, standing on the train platform that October day. I took the northbound train to school, and he went east to work. He’d seemed preoccupied. He didn’t say goodbye when I hurried to board my train.

I’d thought, later, it was because he was planning on leaving that day. Heading back to Earth. But he hadn’t boarded a shuttle. He’d come here.

I felt like I was going to be sick.

I clutched the jacket to myself, wandering blindly through the cavern. I was so nauseous, so overwhelmed, that I almost didn’t see it. But then it was right in front of me, and I couldn’t ignore it. Three meters tall and as wide as four people.

The arch.

I drew a ragged breath. It was here. It was really here. And so was Dad’s jacket. He had the key. He must have used the arch. There was no other explanation. But where had he gone?!

I ran my hands over the rough stones, stacked in their corbeled pattern. It looked just like it had on the coin. Unbelievable.

As my skin touched the stone, the arch seemed to vibrate. There was a hum, barely audible. Like some kind of long-dormant power lay buried inside. Could it really be a tesseract?

A louder sound behind me drowned out the arch’s hum. I froze, listening, though my heart pounded so loudly that I could barely hear anything else.

Voices. In the cave. They were coming.

They’d find me.

I didn’t have time to think. If I’d been thinking straight, I never would have done it. But I was panicking. All I could think of was Joseph Condor on one side—and my dad on the other.

Maybe he hadn’t abandoned me after all.

In a heartbeat, the key was in my hand. I slipped the coin out of my pocket and, fumbling in the dark, found the round recess. Just the right size. Just like Emil said.

I pressed the coin into the divot, and it clicked.

The key was rusty and corroded, but it must have still worked, because when I pressed the coin into it, it sprang open, the hinges along the edges unfolding again and again, from a four-pointed star into a shape too complicated to name. Though its outer casing was metal and worn, the inside of the key was bright and new-looking. Diodes on the interior lit up, glowing purple and emitting a strange, humming whir of sounds, resonating with the sounds from the arch. When I touched one with my finger, it sparked.