Urban centers are often littered with ostensibly public spaces that fall into desuetude because they're not well-planned or a city outgrows them. The "community plaza" that ends up empty or the pedestrian tunnel that turns dark and dangerous 'cause it's not used.
In Berlin, some hockey players are "repurposing" just such a place: an underground walkway below the ICC Congress Center. It's great to see this place put to use–with public health benefits no less.
Here's a Flickr set.