Detection of the 321 GHz water maser with ALMA. Left: Integrated intensity map of 2019.7 321 GHz water masers as detected with ALMA in greyscale. The coloured crosses and triangles show the peak positions of 22 GHz water masers detected in 2017.0 and 2017.8, respectively, while the purple contour is the 5 cm continuum point source (Brogan et al. 2018). The ALMA beam and linear scale are shown at the bottom left and bottom right, respectively. Right: Spatially integrated spectrum of the 321 GHz water masers. The small peaks are HCOOCH₃ thermal line emission, which is close to 321 GHz.