The reliability and precision of an ordinary mechanical watch can be affected by a magnetic field of 50 to 100 gauss. But many scientists are exposed to much higher magnetic fields during the course of their work. Rolex’s solution was the Rolex Milgauss, created in 1956 and resistant to 1000 gauss. Hence the name of the watch, mille being French for thousand.
Please note, there is a waiting list for Rolex watches.