Clearwire (CLWR) is climbing after investment firm Mount Kellett reported that it had obtained 53.2M shares of the company. Clearwire is "substantially undervalued" and has "significant upside potential," the investment firm stated in a letter to Clearwire's board released earlier today. Mount Kellett estimates that it controls 7.3% of voting shares of Clearwire not owned by Sprint (S). In the letter, Mount Kellett also urged Clearwire's board to sell excess spectrum in order to ensure that the company can continue its build-out program beyond one year from now, when the firm estimates that the company's cash is set to run out. If Clearwire fails to take this step, it will be forced to sell itself to Sprint at a distressed price, Mount Kellett contended. In mid-afternoon trading, Clearwire rose 12c, or 6.25%, to $2.12.