  We present the results of ALMA band-5 (~170 GHz) observations of the merging
ultraluminous infrared galaxy, the "Superantennae" (IRAS 19254-7245) at
z=0.0617, which has been diagnosed as containing a luminous obscured active
galactic nucleus (AGN). In addition to dense molecular line emission (HCN,
HCO+, and HNC J = 2-1), we detect a highly luminous (~6e4Lsun) 183 GHz H2O
3(1,3)-2(2,0) emission line. We interpret the strong H2O emission as largely
originating in maser amplification in AGN-illuminated dense and warm molecular
gas, based on (1) the spatially compact (<220 pc) nature of the H2O emission,
unlike spatially resolved (>500 pc) dense molecular emission, and (2) a
strikingly different velocity profile from, and (3) significantly elevated flux
ratio relative to, dense molecular emission lines. H2O maser emission, other
than the widely studied 22 GHz 6(1,6)-5(2,3) line, has been expected to provide
important information on the physical properties of gas in the vicinity of a
central mass-accreting supermassive black hole (SMBH), because of different
excitation energy. We here demonstrate that with highly sensitive ALMA,
millimetre 183 GHz H2O maser detection is feasible out to >270 Mpc, opening a
new window to scrutinize molecular gas properties around a mass-accreting SMBH
far beyond the immediately local universe.
