Possibly Antonie van Leeuwenhoek’s study of microorganisms; he made microscopes that were much better than anyone else’s at the time, and he kept his methods secret and they weren’t properly reverse engineered until the 1950s. (Conventional lens making techniques did catch up, and people like Robert Hooke had been investigating biology on micro scales, but he was probably a generation or so ahead of everyone else.)
Possibly Antonie van Leeuwenhoek’s study of microorganisms; he made microscopes that were much better than anyone else’s at the time, and he kept his methods secret and they weren’t properly reverse engineered until the 1950s. (Conventional lens making techniques did catch up, and people like Robert Hooke had been investigating biology on micro scales, but he was probably a generation or so ahead of everyone else.)