The 60-metre mast on the BMW Oracle Amercia's Cup trimaran has collapsed during testing off San Diego.
None of the crew were injured in the mishap, and support crews are trying to salvage the rigging and the 27 metre boat.
The mast is estimated to have cost around $15 million and is one of three masts the sydnicate has as it prepares to face defending champion Alinghi of Switzerland in a best-of-three showdown starting in February.
The space age-looking craft was relaunched last week after undergoing significant modifications to its main hull. The changes included the installation of a BMW diesel engine to run a hydraulic system that will be used to trim the sails and perhaps move water ballast from one hull to another.
Capable of sailing at more than two times the speed of the wind, they are the fastest, most-extreme boats built in the 158-year history of the America's Cup.