An exemption in the Clean Water Act gives the shipping industry a free pass to dump the seawater stored in ships’ ballast tanks, even though dumping noxious stuff like oils or acids is prohibited. Yet the seawater, picked up in foreign ports, swarms with perhaps the most potent pollutant there is: DNA. It would be hard to design a better invasive-species delivery system than an overseas freighter in the Great Lakes.

The Great Lakes are now home to 186 non-native species. None has been more devastating than the Junior Mint–sized zebra and quagga mussels, two closely related mollusks native to the Black and Caspian Seas. A college kid on a field trip in the late 1980s was the first to discover them in the Great Lakes. In less than 20 years, the mussels went from a novel find to the lakes’ dominant species. If Lake Michigan were drained, it would now be possible to walk almost the entire 100 miles between Wisconsin and Michigan on a bed of trillions upon trillions of filter-feeding quagga mussels. The mussels, which have no worthy natural predators in North America, have transformed the lakes into some of the clearest freshwater on the planet. But this is not the sign of a healthy lake; it’s the sign of a lake having the life sucked out of it.

Adapted from The Death and Life of the Great Lakes