The pilot was departing the turf runway in a tailwheel airplane. As he began the takeoff roll, the airplane "began to pull a little to the left." He corrected with rudder, but the airplane continued to "pull harder to the left." The airplane traveled toward the edge of the runway, where it "dropped suddenly" and exited the runway. The pilot attempted to steer the airplane back onto the runway, and continued to apply full power in order to avoid a large rock pile. He stated that the airplane was "tracking straight ahead" in an area of high brush, and that he decided to "remain with rudder and power rather than questionable braking." The airplane continued through the brush before traveling over an embankment and coming to rest between two trees on the edge of a swamp area. The airplane's wings sustained substantial damage. Postaccident inspection of the runway revealed a 12-inch-deep depression approximately where the airplane exited the runway. Examination of the airplane did not reveal any preimpact failures. The nearest weather reporting station, located approximately 21 miles south of the accident location, reported calm winds at the time of the accident.