  Ultralow-field nuclear magnetic resonance (NMR) provides a new regime for
many applications ranging from materials science to fundamental physics.
However, the experimentally observed spectra show asymmetric amplitudes,
differing greatly from those predicted by the standard theory. Its physical
origin remains unclear, as well as how to suppress it. Here we provide a
comprehensive model to explain the asymmetric spectral amplitudes, further
observe more unprecedented asymmetric spectroscopy and find a way to eliminate
it. Moreover, contrary to the traditional idea that asymmetric phenomena were
considered as a nuisance, we show that more information can be gained from the
asymmetric spectroscopy, e.g., the light shift of atomic vapors and the sign of
Land$\acute{\textrm{e}}$ $g$ factor of NMR systems.
