Chapter 4, ‘Writing It Down: Innovation, Secrecy, and Print’ explains how mining, and subterranean geometry, evolved during the troubled time of the Thirty Years War (1618–1648). It brings together issues related to book history as well as the history of training and teaching practices. Balthasar Rösler (1605–1673) introduced numerous innovations, and his teaching was disseminated by his students among mining regions, in a series of beautifully illustrated and hitherto unstudied manuscripts. The birth of this technical genre is presented in detail, with its evolution and uses within the training system of mining regions. In 1686, Nicolaus Voigtel then published the first practical textbook on the topic. Surprisingly, the craftsmen’s manuscripts weathered the rise of the printed press. I argue that authoring and publishing books failed to supersede the authority of practitioners precisely because their know-how was embedded in a specific technical and cultural setting. Subterranean geometry would stay an underground knowledge for another century, as most innovations arose within this handwritten tradition.