“Quick everyone,” Mr. C said, “look for a message somewhere, something that stands out. Be careful not to trip in the dark.”

“I think I found it!” Chelsea shouted. Everyone hurried over to her. She pointed at the wall. About a foot off the ground, behind a huge boulder, a picture of a stack of books had been drawn in red fluorescent paint. On the floor in front of the picture were tiny glowing stones spelling out, 1905. They only had a few more seconds to look at it before the lights came on and Miss Malone hurried them along. Chelsea glanced back. Without the ultraviolet light, the picture was just a bare wall and the stones were just a design on the ground.

“I think I know where we’re supposed to go!” Gabby said as they emerged into the daylight. “The books must mean a bookstore, or a library!”

“What about the 1905?” Maya asked. “Is it the number of books in the library? The address of the bookstore?”

“It could mean the year the library or bookstore was founded,” Mr. C suggested.

“I don’t know of a bookstore that old,” Gabby said, “but I know of a historic library that might be!”

“Let’s go!” Mr. C said, turning toward the bookmobile.

“Any chance I can dig for a fossil first?” Chelsea asked, only half-kidding.

“We’ll come back, I promise,” Mr. C said.

“Cool.” Chelsea smiled and patted her pocket. Until then she’d have her own little piece of history.