For reasons scientists cannot yet explain, Utah’s Great Salk Lake is laden with toxic mercury. Exactly where the poison is coming from, and how much danger it poses to the millions of migratory birds that feed on the Great Salt Lake, are now under investigation.

Three years ago, tests by the federal Geological Survey showed the lake had some of the highest mercury readings ever recorded in a body of water in the United States. The state warned people not to eat certain kinds of ducks because of the mercury.

The mercury may be accumulating naturally, from some as-yet-unknown source in the ground, or it could be the result of industrial pollution. Mercury released into the atmosphere from coal-fired power plants, gold mines, or industrial facilities may be settling in the lake.