- “Your grandfather had a saying: Our lives are remembered by the gifts we leave our children.”
- ―Preston B. Whitmore
Thaddeus is mentioned by Milo that he acted more like a father to him after his parents' death. He and Milo would often go out camping and tell stories about the lost continent Atlantis. He is also very stubborn, as he kept tempting Whitmore with finding the Shepherd's Journal. Though in his final year, he died heartbroken after years of being the laughing stock by the board members at the museum.
It is known that Thaddeus has been friend with Whitmore for a long time and when Whitmore got fed up about Atlantis existing, they enter into a bet where if Thaddeus finds the Shepherd's Journal, Whitmore will fund the expedition and even kiss him full on the mouth. In 1911, Whitmore funds the expedition that finds the book in Iceland and when he came back it became an embarrassment of the two. But shortly afterward, Thaddeus passes away. According to Whitmore, like Milo, the board members made fun of him. But it is known that he was a friend of Joshua Sweet and told him a phrase 'when you've hit the bottom, the only place left to go is up'.
A comic book prequel story printed as a Disney Festival Special in the Netherlands set before the Iceland expedition, the story opens in Washington, DC, with Thaddeus Thatch reporting his findings of the Journal's supposed whereabouts to Preston Whitmore and making the bet. When research in the Library of Congress results in believing his findings to hold water, he is tailed by thugs that capture him and ransack his home to seize his research, knowing that a job calling for the services of Rourke must be for a incredibly valuable target. After being rescued by the team, he reveals he managed to hide his journal under a statue of Lincoln in a park. When the kidnappers catch up to them, they manage to seize Thatch's journal after holding a member of the team at gunpoint, but he managed to tear out the final page with the location of the Shepherd's Journal during the confusion, allowing the Iceland expedition to continue.
- ↑ http://web.archive.org/web/20190521041805/http://webspace.webring.com/people/gr/rebmakash/LethStamp.HTM