My arms were still crossed, but I have to admit, he caught my interest with “fossil.” Against my better judgment, I found myself paying a bit more attention to Erick’s little spiel.

I might never have noticed it otherwise.

Erick had turned to face the group of us, walking backward and gesturing here and there like some kind of tour guide. “The second reason this site is so interesting is that the entire hills are pockmarked with craters from various meteor impacts over millions of years. The crater walls left behind can give us a snapshot of Mars’ geological processes over time.” He indicated a smallish crater to his right that appeared to have been widened by the dig crew. The sides of the hole were striped, with a variety of different-colored rocks mashed together. It almost looked like Neapolitan ice cream, I thought as I glanced absently down at the bottom of the trench.

Then I froze.

On the crater floor, carefully dug out from the dirt by Erick’s crew, was a pile of smooth-topped stones stacked in the shape of an arch.

I cleared my throat. “Hey, Eri—uh, Professor Gomez,” I interrupted.

He stared for a moment. Then recognition washed over him and he grinned at me. “Oh, Isaak. Did you have a question?”

He seemed so pleased by my interest in his project that I nearly didn’t respond to him. But my curiosity got the better of me. I pointed to the bottom of the pit. “Um, yeah. What’s up with that pile of rocks down there?”

“Oh, that. That’s a natural rock formation. We’ve discovered a number of them throughout this site. We believe they were pushed into that formation by the movement of ancient waterways. Then they calcified, solidifying together like mortar. If you all will follow me, we’ll see a few more of those ‘arches’ on the tour.”

He turned to lead the group through a narrower part of the trail. The rest of my classmates shuffled off after him, but I hung back. Something wasn’t sitting well with me. It may have been a “natural” rock formation, but I’d seen it before—on something that was definitely man-made.

“Isaak, come on,” Tamara called when she noticed I was still hovering at the side of the crater.

I paused for a moment, still looking down at the trench floor. Then, resolved, I turned to her and Henry. “Hang on a sec.” I ducked under the rope surrounding the ditch.

“Whoa, what are you doing?” I heard Henry exclaim as I stumbled my way down the steep, rocky side of the crater.

“I just want to get a better look at it,” I replied.

“Isaak!” hissed Tamara. “You’re going to get in trouble!”

“Then keep a watch for me. Let me know if you hear them coming back.”