About 2.2 pounds of gum is used for every four pairs -- which comes out to around half a pound per shoe. Roughly 3.3 million pounds of gum is unceremoniously applied to the pavement of Amsterdam each year, which means millions of dollars in clean-up costs for the city. The gum in these shoes comes directly from those streets. And appropriately, those streets make their way back onto the shoe: There's a map of the city on its sole.
This works because gum is actually made from synthetic rubber, so "by breaking down these properties, we were able to create a new type of rubber,” Anna Bullus, managing director and designer at Gumdrop, told The Verge.
The project is also hoping to implement a program where the sole can be recycled and replaced once it's worn down -- though you'll have to pay for that service if it gets up and running -- and to expand to other cities.