A large dock - thought to be from last year's Japanese tsunami - has washed ashore on the northwest Washington coastline.
US Coast Guards spotted the 66ft (20m) structure during an aerial search of a remote section of the Olympic National Park.
The authorities are attempting to reach the site and evaluate the dock for any potential invasive aquatic species that might be on board.
The National Park Service has closed the wilderness beach in the area until any risks have been assessed.
They also want to conclusively determine the dock's origin.
Scientists studying tsunami debris from Japan believe it may be one of four pieces from the fishing port of Misawa.
A smaller piece turned up at Newport in Oregon in June.
The Coast Guard had been looking for the large floating debris since a fishing vessel spotted it in the Pacific Ocean last Friday.
"The Coast Guard was out in challenging conditions looking for a needle in a haystack and they found it," Gov Chris Gregoire said in a statement.
The Japanese government estimated that the March 11, 2011, tsunami swept about 5 million tons of debris into the Pacific.
Most of that sank immediately, while 1.5 million tons were dispersed across the North Pacific.
Since then there have been about 1,400 debris reports - mainly small items like bottles and buoys, but also a Japanese "ghost ship" which was sunk by the Coast Guard off Alaska in April this year.
The National Oceanic and Atmospheric Administration estimates the bulk of what is coming either has arrived or will in the next year or so.