During a preflight inspection of the airplane, a certificated flight instructor discovered substantial damage to the underside of the fuselage and reported it to the flight school. Upon further examination, substantial damage to the firewall was also found. Two days before the damage was discovered, the accident airplane underwent a routine phase inspection. The maintenance personnel who examined the airplane for the phase inspection did not report any damage at that time. According to flight school records, there were three flights between the maintenance inspection and the damage report. The first two flights, which occurred later on the same day as the phase inspection, were dual-instructional flights. The third flight, which occurred the next day, and the day before the damage was discovered, was a student pilot solo cross country flight. Interviews conducted by Federal Aviation Administration inspectors with the pilots of those flights failed to reveal who was responsible for the damage, when or where it occurred, or how it was caused. Also unknown, is why the student pilot did not observe the damage prior to his solo flight.