The cave wasn't on any tourist map. Mia found it because she'd wandered off the trail, which she was definitely not supposed to do, and noticed a gap between two boulders just wide enough to squeeze through.
She went back for the others.
There were four of them: Mia, her brother Dev, and two cousins visiting for the summer, Zara and Pip.
Inside, the cave opened into a tall chamber. Their flashlights caught crystals in the walls. The air was cool and still.
"Hello!" Zara shouted.
Hello, hello, hello, the cave whispered back.
"We found it!" Dev yelled.
We found it, found it, found it.
Pip shouted something rude that she immediately regretted. It echoed back twice as loudly.
"It gives back exactly what you put in," Mia said quietly.
She turned to the cave and spoke in a normal voice: "We'll be careful in here."
Careful in here, careful in here.
"We'll leave it as we found it."
As we found it, found it.
"We're grateful."
Grateful, grateful, grateful.
They explored for an hour, careful where they stepped, touching nothing they didn't need to. When they left, they squeezed the boulders back into position.
On the hike back, Pip was quiet.
"That cave made me think," she said finally.
"About what?" Mia asked.
"About how most things work like that. You get back what you put in."
They walked the rest of the way thinking about that.