WITNESSES OBSERVED THE LIGHT, SINGLE ENGINED, AIRPLANE PERFORMING A CLIMB AFTER A TOUCH AND GO LANDING. DURING THE CLIMB WITNESSES REPORTED THAT THE AIRPLANE SUDDENLY PITCHED STRAIGHT DOWN AND DESCENDED INTO THE TERRAIN. AN ON SCENE INVESTIGATION DID NOT REVEAL ANY CONTROL, AIRFRAME, OR POWERPLANT CONDITIONS WHICH WOULD CONTRIBUTE TO SUCH AN UNCONTROLLED DESCENT. THE AIRSTRIP HAD A 3 DEGREE DOWNSLOPE FROM THE CENTER OF THE RUNWAY TOWARD BOTH THE APPROACH AND DEPARTURE ENDS. THE TERRAIN OFF THE DEPARTURE END OF THE RUNWAY DROPS SUDDENLY INTO AN APRX 300 FOOT DEEP VALLEY. INVESTIGATION REVEALED THE AIRPLANE'S LAST ANNUAL INSPECTION WAS 10 YEARS PRIOR TO THE ACCIDENT, THE PILOT'S STUDENT PILOT/THIRD CLASS MEDICAL CERTIFICATE WAS ISSUED APRX 13 YEARS PRIOR TO THE ACCIDENT, AND THAT THE PILOT'S LAST FLIGHT AS PIC WAS APRX TWO YEARS PRIOR TO THE ACCIDENT.