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\section{H_2O Megamasers}  \label{sec:h2o_mm}  Multiple H$_2$O transitions have been observed producing maser emission in galactic source \citep{Elitzur_1992}, however the dominant transition detected as stimulated emission is the 22 GHz transition (see Table \ref{tab:maser_props}). This transition is relatively easy to cause stimulated emission in since it corresponds at a `backbone' transition (lowest levels at each rotational $J$-value). These transitions have large Einstein $A$-values, leading to trapping of the emitted photons \citep{stahler_palla_2004}. This makes collisional excitement and de-excitement more effective than radiation in producing these transitions. The environment needed to produce the 22 GHz H$_2$O emission must pump molecules to fairly high energy levels (640 K, Table \ref{tab:maser_props}). As shown by \citet{stahler_palla_2004}, shocked regions are capable of providing collisional excitement to these levels, and indeed this is where H$_2$O maser emission is observed in both galactic and extragalactic source \citep{Elitzur_1992, lo2005}.  The 22 GHz H$_2$O maser emission is associated withe with  circumstellar material around late-type stars, and with molecular outflows due to young, embedded stars. These environments suggest a collisional pumping mechanism. Mega-maser emission H$_2$O mega-maser emitting regions are typically found within a few parsecs of an AGN, coinciding with regions of dense molecular gas heated to suitable temperatures for masers to occur.  \subsection{Extragalactic versus Galactic Properties}  \label{sub:h2o_props}