The marble disappears inside the structure you built
A CUBORO block can carry a marble in a groove along its top or through a bore hidden inside the wood. The builder has to line up the channel before the ball disappears into the structure. The next visible movement tells them something about the part they cannot see.
In practice. Wooden cubes carry external grooves and internal channels, making route planning a physical puzzle.
What to keep in view
The expansion is designed for a compatible CUBORO set.
Making room for the hidden route
The TUNNEL expansion supplies more ways to route a marble through the interior of a build. It works with the compatible system. A failed run becomes a way to locate a joint that is out of line without taking the whole route apart.
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