At the end of the 19th century, New York City stank. One hundred fifty thousand horses ferried people and goods through the streets of Manhattan, producing 45,000 tons — tons! — of manure a month. It piled up on streets and in vacant lots, and in 1898 urban planners convened from around the world to brainstorm solutions to the impending crisis. They failed to come up with any, unable to imagine horseless transportation.