It was part of a load that recently left on the Interlake Steamship Co.’s freighter Paul R. Tregurtha bound for Detroit’s DTE Electric (MERC is a wholly owned arm of DTE).

MERC brought the Wisconsin complex online in 1976 and transshipped its 100 millionth ton of U.S. western low-sulfur coal in 1991. The 200-million-ton mark was reached in 1998 and 300 million tons was surpassed in 2004. Its most recent milestone, 400 million tons, was recorded in 2009.

It is the largest-capacity coal transshipment facility on the Great Lakes, sending out more coal combined than all other dock facilities in the region.

MERC President Fred Shusterich said the 500-millionth-ton transshipped represents another significant MERC milestone: the loading and transshipment of its 10,000th vessel, American Steamship Co.’s American Century, on November 9.