In article ,
says...

On Thu, 07 Jun 2012 11:34:02 -0400, X ` Man
wrote:

66-foot concrete dock washes ashore in Oregon, may be from 2011 Japan
tsunami

A massive, 66-foot concrete dock mysteriously washed up on the Oregon
shore this week. And officials are trying to figure out if the floating
structure had traveled all the way from Japan after the March 2011 tsunami.

Local affiliate KATU reports that the dock has a placard with Japanese
writing that they are attempting to translate. In addition, the station
traced a phone number on the placard to a business located in Tokyo.

The Oregon Parks and Recreation Department sent a picture of the placard
to the Japanese consulate in Portland for review.

"We don't know where it's from," said Chris Havel with the parks
department. "We don't know if it's from Japan or not but we have to
eliminate those possibilities as we go forward."


http://tinyurl.com/6vdxv6n

Floating concrete surface docks are fairly common. I suppose it could
be from the US but the timing does seem to point to Japan.

I've read articles about how during WW2 they were experimenting with
concrete boats.