Finn and Zadie bent down to see the rock she was pointing to. It wasn’t colorful like the others. In fact, the only color was the cornflower blue Estrella had used for the sky. Underneath was what appeared to be a gray, barren mountain range.

“It’s a mountain,” Finn said.

“No. Look closer.” Luna pointed to several curls of white paint above one of the peaks.

Clouds? No. Smoke.

Finn turned to her sister. “Mom was looking for a volcano.”

The Wilders rose with the blushing dawn. Finn wanted to hit the road early despite having barely slept the night before. When she had slept, she’d dreamt of magma leaping from the mouths of steaming calderas; viscous fire writhing and bubbling under its black crust. When she was jolted awake, it wasn’t from fear, it was from excitement. She’d always wanted to see a volcano, even if it was dormant; even if the only lava she saw had long since cooled and turned to rock.

The only problem was figuring out which volcano was the right one. On the pamphlet display in the front office, she found a Volcanic Parks Map of North America. A little dot identified the location of each volcano: black for extinct, gray for dormant, orange for active, and red for supervolcanoes. Finn had learned about supervolcanoes in geology class. Three of the twenty known on Earth are located in the United States, and according to her teacher, Mr. Mayweather, any one of them would be powerful enough to possibly trigger another ice age. (Mr. Mayweather had a reputation for trying to terrify his students. His lecture on brain-eating amoebas was the reason Finn didn’t swim in lakes anymore.)

Sitting in the open hatch of Zadie’s station wagon, Finn stared down at the map in her lap. There were dozens of dots on the U.S. portion alone. There was no way they could visit all of them. They would have to narrow it down somehow.

Finn reached into her bag and pulled out the egg-size rock Estrella had painted for their mother. Luna had pried it from the Cairn and given it to her in hopes that it would help with their search. Finn couldn’t prove it was her mother’s—although she had floated the idea of dusting it for prints—but she knew it in her gut. Nora’s memory lived somewhere inside Finn. She had felt it. Luna had seen it.

Finn felt the heft of the rock in her palm. It was smooth as a river stone and cool as the water that had shaped it. She felt silly for having thought it might be warm to the touch, as if the volcano itself were somewhere inside, its smoke venting through the tiny pores in the rock’s surface.