I colored, but nodded. He was right. Though Abuelo had always said there was a fine line between crazy and genius.

When I was done looking through the second book, I said, “So where does the coin fit into this? And the key? What is the key, anyway?”

Emil moved back over to the bookcase. “The coin and the key are, I believe, the answer to all of this.” He brought another paperback over. This one wasn’t a romance. It was an old sci-fi by Robert Heinlein—Podkayne of Mars. He opened this and flipped forward several pages to a drawing of an arch identical to the engraving on the coin. A human figure passed under the vault.

He put an ink-stained finger on the drawing and said, “I’m sure you noticed a lot of talk about Atlantean Arches on those websites of yours, right?”

I nodded.

“They’re found in most major civilizations on Earth. But I’ve found them here on Mars, too. There is one exceptionally well-preserved example not far from here.”

“At Erick’s dig site?” I asked.

“No, no. Closer. There are a series of caves just past the factory district. I believe the first colonists used these caves for shelter when they came here.”

I stared at him. “The first colonists?”

“Yes, of course. Clearly the presence of these artifacts on both planets indicates prior planetary exploration by ancient humans.”

“Wait, wait, wait.” I held a hand up, trying to process his words in my brain and coming up blank. “You think people from Earth left these things thousands of years ago?”

“Of course,” Emil said. “Surely you don’t think that aliens left them, do you?” When I didn’t answer, he grumbled, “Why is it always so difficult for people to believe in the power of the human mind? Why is it so easy for us to assume that some other, smarter species did it? Is it because we can’t bear the thought of previous generations being wiser than our own?”

“But how did they get here?” I said.

Emil grinned and tapped Podkayne. “The arch,” he said simply.

I looked down at the diagram, the person walking through the archway. “So it’s, like… a door?”

“Exactly. Didn’t you ever read any books growing up? I think this arch is a tesseract.”

Tesseract. That was a word I had heard before. Of course, I wasn’t going to tell him that I hadn’t read it in a book—I’d picked it up from one of Henry’s old sci-fi flix. It was some kind of object that existed in multiple dimensions. Supposedly you could use it to travel across vast distances by bending space.

“So the coin and the key…?”