Author Dan Egan establishes the beauty of the Great Lakes of Michigan, as well as their unique properties; each Great Lake is more like an ocean due to its size. The Great Lakes contain 20% of the world’s freshwater, which is significant considering that 97% of the world’s water is saltwater, and millions of people around the world still lack access to clean to drinking water. Egan sets up the story of how this once pristine body of water became tainted due to industrial pollution in the 20th century—so much so that the nearby Cuyahoga River in Cleveland burst into flames.

The tragedy of the Great Lakes is compounded even further because of another environmental disaster: the infiltration of biologically contaminated water from the ballasts of overseas freight ships disrupts the ecology of the Great Lakes. Despite the passage of the Clean Water Act in the 1970s following the 1969 fire on the Cuyahoga River, the U.S. Environmental Protection Agency (EPA) allows an exemption for this biologically contaminated water to pass into the Great Lakes to avoid problems for the shipping industry.

Egan describes how the discharged ballast water—up to 10 Olympic swimming pools’ worth—from one of these freight ships is an unintentionally perfect way to dump invasive species into the Great Lakes.