The airplane was in cruise flight at 29,000 feet in smooth air with the seat belt sign turned off. The crew reported two jolts, and the airplane descended 600 feet. Due to possible injuries, the crew diverted to Bermuda (BDA) and landed. Readout of the flight data recorder indicated cruise flight at 29,000 feet with no change in heading, airspeed, or altitude for several minutes approaching the accident sequence. The altitude graph showed a slight climb before the airplane descended to 28,400 feet. The graph line indicated a slow recovery to 29,000 feet with no oscillations. The elapsed time between the first altitude excursion to recovered cruise flight was approximately 75 seconds. Of the 155 passengers on board, one passenger received serious injuries and 67 received minor injuries. The flight was scheduled for Flight Level 330 to Flight Level 370. Therefore, no information concerning forecast turbulence at Flight Level 290 was available to the flight crew. The High Level Significant Weather Prognosis chart for that period revealed that no significant weather was shown in the area where the turbulence was encountered. Probable Cause: the aircrew’s inadvertent encounter with unforecast clear air turbulence.