The disturbing news is that over 200 million of the 1.3 billion gallons of used oil generated in the U.S. yearly are not collected, but rather dumped illegally or accidentally into sewers, streams, drains, landfills and backyards:

• 200 million gallons is the equivalent of nearly 20 Exxon Valdez spills each year.
• One gallon of used oil can contaminate up to one million gallons of fresh drinking water.
• One quart of used oil can create a two-acre slick on surface water.
• 40% of the pollution in U.S. waterways is from used motor oil.

Sources:
American Petroleum Institute (API)
National Oil Recyclers Association(NORA)

What happens to your vehicle's used motor oil after it's collected? Good question.
Consider the following chart: