Otto Hillig was a Danish machinist who helped work on various projects in the early days of the Manhattan Project. Nicknamed the “Great Dane” by Enrico Fermi, Hillig proved valuable in multiple capacities while working on Chicago Piles 1 and 2. Hillig served as a carbon dioxide moderator during several experiments on Chicago Pile 2. He had a friendly relationship with both Fermi and Walter Zinn, oftentimes playing jokes on them.