There are images of upside-down buildings hidden in city skylines

Artist Peter Wegner photographs the negative space between buildings in New York, Chicago, and San Francisco, then turns them upside-down, revealing an invisible city made out of sky.

Wegner's Buildings Made of Sky series looks at the unintentional spaces between the intended ones, and creates something fantastical in the process. You can see more from the series at his website.

Buildings Made of Sky [Peter Wegner via Fast Co.Exist]