“The AER estimates over $45 billion in remediation and reclamation liabilities in the oilsands. This number may be a dramatic underestimate, with figures in leaked, official presentations suggesting as much as $130 billion in liabilities covered by less than $2 billion in security deposits,” University of Calgary School of Public Policy researchers Martin Olszynski, Andrew Leach, Drew Yewchuk wrote in a recent paper.
Just to add another one, the Sydney Tar Ponds covered 102 hectares (1.02 km^2) and cost $400M (2013) to clean up.
One of the Tailings ponds I’m familiar with is 9 km x 6 km in size…54km2