Spain

In Barcelona, tourists walk around on a giant sand filter.

The great square in Park Güell, framed by Gaudí’s winding mosaic bench, is covered with almost 3,000 square metres of coarse sand. The sand is not just a floor. It filters the rainwater.

The water seeps down through the sand and is led through hidden channels to a cistern beneath the square, where it is stored to water the park. Even the famous mosaic dragon by the stairs is part of the system: when the cistern is full, the water runs out through its mouth.