The "dead zone" off Oregon's coast is back -- larger, thicker, and more lethal than ever.

For the fifth year in a row, scientists have witnessed thousands of sea creatures dying in the Pacific Ocean.

This year, the dead zone covers 1,200 square miles, according to Oregon State University marine ecologist Jane Lubchenco.

On Tuesday, Lubchenco's team witnessed crabs dying by the thousands.

kgw.com image/courtesy OSU

OSU scientists found thousands of dead and dying crabs in the "dead zone" off the Oregon coast.

"There are no fish down there that we could see," Lubchenco said. "This is an area where we have measured chronically low oxygen."

Undersea video taken Tuesday shows thousands of dead and dying crabs.

Scientists found almost no oxygen in the water there. According to OSU marine ecologist Francis Chan, "we're only half-a-step away from zero -- or the absence of oxygen."

The dead zone now ranges from Florence to Lincoln city, and for the first time, there's another pocket off Washington's Olympic Peninsula.

While other ocean dead zones have been caused by pollution, scientists say they're not sure what's causing the current die-off. The leading theory: global warming.

"What we're seeing is absolutely consistent with predictions for climate change," Lubchenco said, "but we cannot say for sure that's what causing it."

As the dead zone becomes a regular occurrence, researchers say it points to a fundamental shift in marine conditions and ocean behavior.

OSU researchers are now working on new models to allow scientists to predict when the dead zones will occur.