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Although water may be abundant in molecular clouds, measurements of the abundance are difficult because atmospheric absorption prevents access to transitions between low-lying levels. Highly excited transitions are observable from the ground, but they sample only hot and dense gas and are usually masing, and are not useful for deriving the abundance. We have detected the 547~GHz $1_{10} - 1_{01}$ ground-state transition of ortho-$\rm H_2{}^{18}O$, using our submillimeter SIS receiver flying aboard the NASA Kuiper Airborne Observatory. The 547~GHz line is in absorption towards Sgr~B2 and is in emission in OMC-1. For Sgr~B2, the source structure and radiative transfer modeling lead us to conclude that we are sampling the cool, quiescent molecular envelope rather than the hot core region. For ortho-$\rm H_2{}^{18}O$, only the $1_{01}$ ground state is expected to have a significant population in this envelope. We derive an $\rm H_2{}^{18}O$ column density of $1.1 \times 10^{14} \ \rm cm^{-2}$, which gives $\rm H_2{}^{18}O / H_2 \approx 7 \times 10^{-10}$ and $\rm H_2O / H_2 \approx 1.8 \times 10^{-7}$, assuming $\rm {}^{16}O / {}^{18}O \approx 250$. The velocity width of the 547~GHz emission line in OMC-1 is consistent with the $\rm 20\ km\ s^{-1}$ outflow which is associated with 22~GHz maser emission, indicating that we may be sampling shock-excited gas. We have also made a tentative detection in OMC-1 of the $2_{11}-2_{02}$ transition of para-$\rm H_2{}^{18}O$ at 745~GHz. This work was supported in part by grants from NASA (NAG2-744 and NAGW-107), NASA/JPL, and an NSF Presidential Young Investigator grant to J. Zmuidzinas.
