Maybe you've seen them: white bicycles chained to signposts and fences, perhaps even in your neighborhood. They're known as ghost bikes, and serve as somber memorials of tragedy.

Each one symbolizes a life lost while on a bicycle.

New York, and more than 200 other cities around the world, are dotted with these unassuming yet emotionally jarring bicycles. Spray-painted entirely white from the handlebars to the hubs, the bicycles have a tendency to blend into the cityscape.