  Copper nuclear quadrupole resonance (NQR) and zero field nuclear magnetic
resonance (ZFNMR) studies of YBa$_2$Cu$_3$O$_{6.5}$ show that a magnetic phase
appears in underdoped 123 superconductors treated in ambient wet air. The
studies give convincing evidence that the ``empty'' CuO chains play the role of
easy water insertion channels. The reaction occurs first in ordered regions of
the crystallites. The final product of the reaction is a non-superconducting
antiferromagnetic compound characterized by at least two types of magnetically
ordered copper ions with ZFNMR spectra respectively in the frequency ranges of
46-96 and 96-135 MHz respectively. Even for powder samples fixed in an epoxy
resin, the reaction is found to occur partly after a few years.
