In March 2011, the franc climbed past the US$1.10 mark (CHF 0.91 per U.S. dollar). In June 2011, the franc climbed past US$1.20 (CHF 0.833 per U.S. dollar) as investors sought safety as the Greek sovereign debt crisis continued. Continuation of the same crisis in Europe and the debt crisis in the US propelled the Swiss franc past US$1.30 (CHF 0.769 per U.S. dollar) as of August 2011, prompting the Swiss National Bank to boost the franc's liquidity to try to counter its "massive overvaluation". "The Economist" argued that its Big Mac Index in July 2011 indicated an overvaluation of 98% over the dollar, and cited Swiss companies releasing profit warnings and threatening to move operations out of the country due to the strength of the franc. Demand for francs and franc-denominated assets was so strong that nominal short-term Swiss interest rates became negative.