Add this question to the many mysteries of Iliamna Lake: Where the heck do its unique, freshwater harbor seals go when the lake and rivers freeze shut?

Dave Withrow, a federal research biologist who has flown numerous aerial surveys to count them, has spotted close to 300 of the seals in late summer.

In deep winter when there are no openings in the ice, he's counted as many as 73. During other such times, he's found none. Locals from nearby villages say the seals live there year-round, and based on what he's seen, he believes them.

Which begs the question: "How can 280 seals just disappear?"

Maybe the seals have a hidden cave where they winter, he once joked with colleagues. Then an elder told him just such an underground cave existed, providing freshwater access for the seals year-round, a legend that gets a mention in a recent article by a state Fish and Game biologist.

"We were just totally joking, but to hear an elder say that was like, 'Really? I don't believe it,'" Withrow said.

Welcome to Alaska's deepest and most massive lake, some 200 miles southwest of Anchorage across Cook Inlet.

The seals, hunted by Alaska Native villagers from communities like Kokhanok on the lake's southern shores, are fatter than seagoing harbor seals, and have thicker fur. The hunting was first documented by a Russian explorer in 1819, according to the article.

The animals are just one of two such harbor-seal populations in the world, and Withrow hopes to determine if they're a genetically distinct stock. But they're just one of the lake's mysteries. There's also the strange white beast that's said to occasionally ripple the surface, a la the Loch Ness Monster.

An Alaska scientist thinks the "Iliamna Lake Monster," as some locals call it, might be sleeper sharks that spend much of their time dwelling on the bottom of the lake, occasionally rising to the surface to feed.

Bruce Wright, the scientist who led the Exxon Valdez restoration effort for the federal government, hopes to catch a sleeper shark this summer after capturing them on video with a waterproof camera lowered in a cage.

But, in a lake covering 1,600 square miles and 1,000 feet deep, he has to narrow down the search. To learn where the sharks might lurk, he's trying to figure out what they eat. Maybe they feast on huge runs of red salmon that surge 75 miles up the Kvichak River after leaving Bristol Bay. Maybe they chomp on the seals, ripping out their midsections with a form of night vision that senses electromagnetic fields when animals move.

Legend also has it that the bottom reaches of the lake are filled with saltwater where the sharks can dwell year-round, and that maybe there's some sort of connection to Cook Inlet. It sounds like something out of McElligot's Pool in the Dr. Seuss classic of the same name. Anything's possible in McElligot's Pool, and who knows, maybe it's the same for Iliamna Lake. At any rate, Wright plans to check out the saltwater theory, too, lowering a meter that can detect salinity.

Withrow isn't convinced of the sleeper shark theory, though he admits he can't disprove it either.

He's conducted about six aerial surveys per year since 2007, and he's never seen a sleeper shark from the air in the lake. Growing more than 20 feet long, they'd be visible in the lake's clear water. But that doesn't mean they're not there. Maybe they don't bother with the seals because there's enough fish to keep them full.

Withrow believes it's more likely the baffling creature is a sturgeon, a prehistoric fish with a long snout. They grow several feet long and are known for swimming up rivers and into lakes, a feat that in the shallow Kvichak River would be tough for a sleeper shark.

Or, maybe the sturgeon, or some other beast, has been there since the last Ice Age, waiting to be discovered, he said.

The sturgeon concept jives with what Chad Anelon of Iliamna, a village on the lake's north shore, has always heard. He's never seen the lake monster, but he's talked with others who have.

They say "it looks like a sturgeon," he said.

As for the lake's seals, Withrow expects to study them in greater detail soon. He works for the National Oceanic and Atmospheric Administration, but it's been a long process getting permits from the agency even to collect bits of samples from Native hunters, who enjoy an exemption for hunting those seals and other animals under the Marine Mammal Protection Act. When those permits come through, as they soon should, Withrow also hopes to radio-tag the seals to track their routes. Then he just might learn their winter whereabouts.