Fortescue has about $US12 billion of gross debt sitting just beyond the horizon, and will have to repay that over a nine-year period in which iron ore prices are universally tipped to decline. Fortescue's highest-profile critic, New York hedge fund manager Jim Chanos, believes that combination of factors could create a ''witches' brew'' that brings Fortescue to its knees. ''This is a company that has made a one-way bet on one commodity,'' he told Fairfax Media this month in a rare interview. ''The iron ore miners have made a big bet that demand is going to continue increasing meaningfully in China.''