Video obtained by KIRO 7 Eyewitness News shows scientists exploring giant mounds at the bottom of Hood Canal.
The scientists were concerned the mounds were harming the underwater environment by stealing the oxygen supply.
The mounds formed several hundred feet below the canal's surface, just west of Dewatto Bay and Little Dewatto Bay in southern Hood Canal.
State Department of Natural Resources geologist Dave Norman was part of a team of scientists who went deep-sea searching for mystery mounds.
They sent a small, remote-controlled vehicle into the water -- the only practical way to get a good look.
What the high-resolution camera sent back was eye-opening to the team.
The dirt and rock makes up two giant formations in southern Hood Canal, each more than a 100 feet tall and more than a 1,000 feet wide.
After filming for four hours, scientists took a close look at the video, and they finally knew how the mounds formed.
"These are landslide deposits that came off of the deltas," said Norman. "These landslides slid off the side of Dewatto Creek and were deposited into the depths of Hood Canal."
What scientists don't know yet is how the landslides formed. They said what's happening beneath Puget Sound might help them figure that out.
"It's possible an earthquake triggered these landslides, but it's also possible it didn't as well. We don't know for sure," said Norman.
They said the mounds aren't hurting the environment; they're helping it. And they don't know how long the mounds have been here.
Answering that question will take longer, and more future deep-sea exploring.
Scientists plan to use the underwater vehicle next summer in Puget Sound, looking at more fault lines.