Minnesota Public Radio slideshow:
What's behind the mystery rust
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This piece of steel was removed from an underwater wall in the Duluth-Superior harbor. The steel is covered with round pits. The pits, caused by corrosion, would eventually rust through the steel, weakening the wall and potentially causing premature failure. (MPR Photo/Bob Kelleher)

This piece of steel was removed from an underwater wall in the Duluth-Superior harbor. The steel is covered with round pits. The pits, caused by corrosion, would eventually rust through the steel, weakening the wall and potentially causing premature failure.

(MPR Photo/Bob Kelleher)
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