The atavachron was a time travel device from the planet Sarpeidon. The device was capable of sending individuals back into the planet's past at any point. When James Kirk, Spock, and Leonard McCoy beamed down to Sarpeidon to investigate the lack of people on the planet, they were able to determine that, knowing their planet would be destroyed, Sarpeidon's inhabitants fled into the past. Since the device sets their cell structure to the era they enter, attempts to return to the future would instantly kill them. Kirk, Spock, and McCoy were accidentally transported through time by accident, but were recovered. (TOS episode: "All Our Yesterdays")