Re-ball-track transforms the dupont underground into an interactive ball track for young and old visitors. The approximately 200 meter long ball track invites you to dive into the deep underground into a blue world with a mystic cloudy sky out of thousands of plastic balls. As you enter you pick up one of the recyclable, hypoallergenic plastic balls out of the cloud. You are prompted to play with your ball, solve problems to get further and accompany it, till you reach the illuminated end of the ball track, where all balls once put on the track gather together on a big pile on the ground. Here the ball out of a cloud become a raindrop that ends as part of the sea again. The «re-ball-track» project let trickle the 650‘000 balls from The Beach at the National Building Museum trough the ceiling into the dupont underground, where they bulge into a big cloud. It references to the dupont surrounding and its historic role as a traffic tunnel where something was brought from A to B while it converts its original function to a playful and interactive ball track. re-ball-track also links to The Beach while it creates at one end of the tunnel a bright, calm, relaxing space. This relaxing space again references to the park on the upper level. The balls are held by a thin and flexible net at the ceiling with a gap at one point that let the balls through. The ball track will be made by a suspended wooden slats structure. There are seven mechanical machi- nes incorporated into the track that let users can interact with the balls. Spotlights at the floor create a blue underground atmosphere in the tunnel and transfer the end of the ball track into a white shining space where visitors are invited to plunge into the balls again.