The flight instructor and student pilot were practicing approaches off-airport in the helicopter, and the flight instructor was demonstrating a slope landing. The flight instructor described that while in the hover, the helicopter suddenly pitched up 10-20°, followed by a pitch down of a like amount. The main rotor subsequently impacted the ground and the helicopter rolled over onto its left side, which resulted in substantial damage to the tail boom, main rotor, and fuselage. The flight instructor reported that there no preaccident mechanical malfunctions with the helicopter that would have precluded normal operation. Photographs of the accident site showed that the surface the hover was being performed over was an open area of uneven gravel. In a statement to a law enforcement officer who responded to the accident, the flight instructor described that the helicopter’s skid had contacted the gravel and the helicopter then flipped over. Given this information, it is likely that during the hover, the helicopter inadvertently contacted the gravel surface, which initiated a dynamic rollover event from which the flight instructor could not recover.