In 1904, writer Upton Sinclair - a so-called "muck-raking" journalist - went undercover at the Union Stock Yards in Chicago to investigate and expose the working conditions imposed upon the employees of the city's meat-packing industry. What Sinclair discovered shocked the nation.

In addition to unsanitary, cruel and dangerous conditions for the workers, Sinclair also discovered that the meat being shipped out from the Stock Yards was often rotten and contaminated, with rancid meat being stuffed into sausages rather than discarded. Immigrant workers and poor native Americans alike were often exploited, forced to work at highly dangerous, menial and deadly jobs, exposed to dangerous chemicals and subjected to long hours in airless, fetid factories.

Sinclair fashioned these stories into "The Jungle," a fictional but searingly realistic account of a young Lithuanian immigrant who finds himself in Chicago seeking his fortune, only to be thrust into a horrifying new existence as a worker in the Stock Yards.

The book caused an immediate sensation upon its release and the accounts of worker exploitation - but especially the stomach-churning details of how meat was produced in America - led to vast social and political reforms, including the Meat Inspection Act and worker protections.

"The Jungle" stands as one of the most important and influential books of the early 20th century and the reverberations from its release can still be felt today.