GREAT SALT LAKE, Utah —The Great Salt Lake is so briny that swimmers bob in the water like corks. It is teeming with tiny shrimp that were sold for years in the back of comic books as magical “sea monkeys.” And, for reasons scientists cannot explain, it is heavily 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.

“We’ve got a problem, but we don’t know how big it is,” said Chris Cline, a U.S. Fish and Wildlife Service biologist who has been collecting the eggs of cinnamon teal ducks from nests along the rim of the lake so that they can be cracked open and analyzed in the lab.

Three years ago, in an alarming finding, U.S. Geological Survey tests 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.

This summer, scientists are fanning out across the lake and its marshy shoreline for the start of what is expected to be a multiyear study. The Environmental Protection Agency and the state are footing most of the $280,000 bill for the initial phase.

One major question is whether the mercury is accumulating naturally, from some as-yet-unknown source in the ground, or is the result of industrial pollution. Researchers say mercury released into the atmosphere from coal-fired power plants in the West, gold mines in Nevada, volcanoes in Indonesia or industries in rapidly developing countries such as China or India may be settling in the lake.

Mercury can cause neurological damage in birds and affect their ability to fight off diseases. High mercury levels have been detected in some of the Great Salt Lake’s birds. But so far there is no evidence that it is sickening them.

“The jury’s kind of still out on the impact, but it can’t be a good impact,” said Tom Aldrich, migratory gamebird coordinator for the Utah Division of Wildlife Resources.

For all of its international recognition, the Great Salt Lake is still a deep source of mystery. The lake is the remnant of a sprawling prehistoric inland sea that covered 20,000 square miles during the last ice age and was 1,000 feet deep in places.

Today’s Great Salt Lake is much smaller at 1,700 square miles, much shallower and significantly saltier — brinier even than the ocean — because of salts dumped by tributaries and left behind as the water evaporates.

Businesses make millions of dollars selling tiny brine shrimp, salt and other minerals from the lake, and it is a popular spot for boaters and other tourists, despite the often strong rotten-egg smell from decaying algae and other organisms, and despite the fact that practically the only creatures that can survive the salty water are bacteria, bugs and the shrimp. (Scientists say the mercury poses no danger to swimmers.)

But the wetlands and wide expanses of calm water are a powerful draw for birds, including the world’s largest concentration of Wilson’s pharalope, which uses its needle-like bill to gobble shrimp and bugs; the eared grebe; the white-faced ibis; the California gull; and the snowy plover.

Each year, more than 9 million birds stop by, many on their annual treks between Canada or South America and parts between, making the Great Salt Lake “sort of the Delta airplane hub of the West in terms of migration,” Aldrich said.

The problem is that the lake has a peculiar combination of bacteria and chemicals that helps convert inorganic mercury to its more harmful form, methylmercury, scientists say. In some cases, the mercury gets into the brine layer on the lake bottom and makes its way up the food chain to the shrimp and then to the birds that eat them.

Once they have identified the source of the contamination and what sort of threat it poses, scientists hope to somehow reduce the amount of mercury getting into the lake, or perhaps control the bacteria that lead to the more harmful form.

“Hopefully something can be done,” said Jodi Gardberg, the state’s Great Salt Lake watershed coordinator.