The flight instructor and the student pilot were on a multi-leg cross-country flight, and conducted an overflight of the private-use airport’s 2,400-ft-long by 100-ft-wide turf runway to assess its condition. They observed no obstacles or wildlife and landed uneventfully. After performing a preflight inspection of the airplane, the flight instructor planned to perform a soft-field takeoff. The flight instructor reported that during the takeoff roll, it felt as if the left main landing gear struck something, after which the airplane suddenly began a hard turn to the left. The flight instructor applied full right rudder and reduced engine power in an attempt to regain control; however, the airplane exited the runway and struck water adjacent to the runway. The airplane sustained substantial damage to the elevator during the accident sequence. The flight instructor and the student pilot reported there were no preimpact mechanical malfunctions or failures of the airplane that would have precluded normal operation. An FAA inspector who examined the runway after the accident observed noticeable areas of uneven ground, as well as various low and high spots that were the result of ant hills, mole tunnels, and ground settling. Given this information, it is likely that one of the airplane’s main landing gear briefly contacted one of these areas of uneven ground during the takeoff roll, which resulted in the airplane’s left turn and the subsequent runway excursion.