The Zeeman effect describes the splitting of spectral lines in magnetic fields. This effect could be observed in the water molecule (H2O) for the first time with the 100-m telescope and allowed the study of magnetic fields in molecular clouds.
D. Fiebig and R. Güsten 1989, "Strong magnetic fields in interstellar H2O maser clumps", Astronomy and Astrophysics 214, S. 333-338.