WINDSOR, Ontario — Assumption Park gives residents of this city lovely views of the Ambassador Bridge and the Detroit skyline. Lately they’ve been treated to another sight: a three-story pile of petroleum coke covering an entire city block on the other side of the Detroit River.

Detroit’s ever-growing black mountain is the unloved, unwanted and long overlooked byproduct of Canada’s oil sands boom.

And no one knows quite what to do about it, except Koch Carbon, which owns it.

The company is controlled by Charles and David Koch, wealthy industrialists who back a number of conservative and libertarian causes including activist groups that challenge the science behind climate change. The company sells the high-sulfur, high-carbon waste, usually overseas, where it is burned as fuel.

The coke comes from a refinery alongside the river owned by Marathon Petroleum, which has been there since 1930. But it began refining exports from the Canadian oil sands — and producing the waste that is sold to Koch — only in November.

“What is really, really disturbing to me is how some companies treat the city of Detroit as a dumping ground,” said Rashida Tlaib, the Michigan state representative for that part of Detroit. “Nobody knew this was going to happen.” Almost 56 percent of Canada’s oil production is from the petroleum-soaked oil sands of northern Alberta, more than 2,000 miles north.

More: Mountain of Petroleum Coke From Oil Sands Rises in Detroit