“Hmmm,” Jaime said. “What’s that?”

“What?” said Tess.

“That,” said Jaime, pointing. A few feet in front of them, something crowded the tunnel. They stopped short, but Nine wasn’t having it. She jerked her leash out of Tess’s hand.

“Wait! Nine!”

Nine didn’t wait. She bounded forward, nearly swallowed up by the darkness. Her stripes and spots seemed to float in the air, her snuffles echoing off the stone. After a few moments, she chirped softly, calling to them.

“Wait!” said Theo, but Tess felt along the walls till she reached the large something that filled the tunnel. Which, when she ran her hands over the wheels and the sides, turned out to be some sort of carriage, the kind that you’d see in Central Park, drawn by sleepy, clopping horses. But when Tess peered around to the front of the carriage, there were no horses hitched there. There wasn’t anything hitched there. Unless? She felt along the reins, which tapered as they stretched all the way to the ground, where there were . . .

Tess jumped back.

“What?” said Theo.

“Ants,” said Tess.

“Ants?” said Theo.

“Four mechanical ants. They’re each about the size of a thumb,” said Tess.

“Are they moving?”

Tess crouched and squinted in the gloom. The antenna of one ant was swiveling their way, as if listening. “Maybe?”

Jaime crouched next to her. “Yep. Ants. Because why not ants?”

“The real ones can carry more than a thousand times their body weight,” said Theo. “Who knows what the mechanical ones can carry?”

“A fully loaded carriage, I’m guessing,” said Jaime. “Shall we?”

They got into the carriage. As soon as they’d sat down, the ants began to march forward. Or rather, scuttle. Despite the scuttling and the cobblestone tunnel floor, the ride was surprisingly smooth. Nine the Cat positioned herself at the front of the carriage, letting the slight wind ruffle her whiskers, like the captain on the prow of a ship.

They didn’t scuttle far, however. Or at least, the ride didn’t seem to be that long. But they did pass other openings in the tunnel, lefts and rights they didn’t take. Who built this tunnel? How did the ants know where to go?

The ants weren’t saying.

Tess wished she could talk to her grandpa Ben, wished he was here, right now, riding along with them. But Grandpa Ben had been moved from 354 W. 73rd Street to a place on Long Island, and to a fancy facility uptown immediately after that. Grandpa Ben forgot more than he remembered.

And then, as suddenly as they had begun, they stopped moving.

“We’re here,” said Tess.

“Great. Where’s here?” Theo said. The ants’ antennae twitched, seemed to point to a set of rough stone steps chopped into the nearest wall.