During a cross-country flight under instrument flight rules (IFR) the pilot was cleared from 9,000ft to 7,000ft over mountainous terrain. In the descent, the pilot reported encountering instrument meteorological conditions (IMC) and extreme turbulence which caused a loss of control resulting in inverted flight. The pilot was able to recover from the loss of control, but was unable to recall how he recovered. After the upset and recovery, the pilot landed without further incident. According to information obtained from a Safety Board meteorology specialist, weather data displayed a band of clouds in the immediate vicinity of the upset location associated with mountain wave activity and locally generated turbulence. Another aircraft in the immediate vicinity also reported an encounter of moderate-to-severe turbulence prior to the event. There were no AIRMETs or SIGMETs in effect in the area where the turbulence was encountered. The pilot reported no mechanical malfunctions or failures with the airplane that would have precluded normal operation. During a post-flight inspection, both wings were substantially damaged after wrinkling and tearing was found.