River pebble underground puzzle

Quick answer：

River pebble underground puzzle refers to the use of river pebbles to create a puzzle-like arrangement underground. This can be an artistic installation or a landscaping feature. River pebbles are naturally occurring stones found in riverbeds, and they are often used in landscaping for their aesthetic appeal. In this context, the stones are arranged in a specific pattern or design, creating a puzzle that can be solved or admired.