Scientists theorize that two mysterious mounds on the bottom of Hood Canal were caused by underwater landslides.

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Washington scientists guessed that mysterious mounds hundreds of feet below the surface of Hood Canal were deposited by ice age glaciers or built up by natural-gas seeps or geothermal vents.

After taking a closer look with a remote-control camera they have another theory: Underwater landslides.

There are two dome-shaped mounds, each more than 100 feet high and more than 1,000 feet wide, on the bottom at the south end of the canal near Lilliwaup, Mason County.

Scientists with the Department of Natural Resources and the Department of Fish and Wildlife used a submersible vehicle in 2008 to take a closer look. They decided the mounds were created by underwater landslides of sediments that had built up at the mouths of two small rivers.

The study has been discussed at conferences, but budget cuts were one reason the report wasn’t published until November by the Division of Geology and Earth Resources.

Department scientists first noticed the mounds in sonar maps made by the University of Washington in the canal, a 68-mile fjord on the west of Puget Sound.

“We were left scratching our heads,” said Michael Polenz, a Department of Natural Resources (DNR) geologist involved in mapping the canal for possible hazards.

They were able to rule out glacial deposits, which are usually ridge-shaped. And there was no sign of seeping methane or discharging geothermal vents.

The location was the best clue to the origin of the mound. One is off the mouth of the Duwatto River and the other off the mouth of the Little Duwatto River.

They are the only rivers on the Hood Canal without deltas, said Tim Walsh, chief hazards geologist with the DNR.

It’s assumed the unstable deltas slid to the bottom of Hood Canal, perhaps in an earthquake about 1,000 years ago. Similar underwater landslides today could cause a tsunami on the canal, depending on the speed of the landslide.

“We have a lot more questions than answers,” Walsh said Friday.