The dead sea otters arrived at Melissa Miller's Santa Cruz, California, lab with bright-yellow eyes and gums, their livers destroyed.

One by one, Miller, a marine-wildlife veterinarian, eliminated the potential causes of death until "the last thing I was left with seemed so implausible that I thought I was going crazy."

The otters had been poisoned by a "nasty toxin" called microcystin, which is produced by cyanobacteria, or blue-green algae. Such toxins can appear when human sewage and fertilizers run into lakes and rivers, adding nutrients that spur the growth of algae "superblooms," Miller said.

But sea otters stick to the ocean, never entering the polluted lakes and rivers where these blooms occur.

"I said, OK, we have to figure out how the otters are getting into this," said Miller, of the California Department of Fish and Wildlife and the University of California, Davis.

Miller's sleuthing led her to California's Pinto Lake (map), a water body about 2.5 miles (4 kilometers) from the ocean and so prone to superblooms that Miller said "it'll blow your mind—it looks like fluorescent green paint."

Sure enough, she found that Pinto Lake eventually drains into the Pacific Ocean—close to where the dead otters were found in 2007.

Later experiments revealed the algae's toxins can live for long periods of time in shellfish—otters' main diet. Toxins from the polluted lake were traveling downstream into the ocean, Miller concluded, where they were getting into shellfish and killing otters.

This article is based on a research paper by the National Geographic and is free to be used for information purposes, under the CC. I feel that all of us should read and understand the article to get deeper knowledge about what is actually doing the damage.

The image is also owned by the Nat Geo - under CC License.