The Avatar and the Firelord - S3-E6
Firelord Sozin: With Roku gone and the Great Comet returning, the timing was perfect to change the world. I knew the next Avatar would be born an Air Nomad, so I wiped out the Airtemples, but somehow the new Avatar eluded me. I wasted the remainder of my life searching in vain. I know he's hiding out there, somewhere. The Fire Nation's greatest threat; the last Airbender.
Chosen answer: Toph had Katara use it to send a letter to her parents. It never returned.