We report the results from a 19-h integration with the Spectral and Photometric Imaging REceiver (SPIRE) Fourier Transform Spectrometer aboard the Herschel Space Observatory which has revealed the presence of a molecular outflow from the Cosmic Eyelash (SMM J2135−0102) via the detection of blueshifted OH absorption. Detections of several fine-structure emission lines indicate low-excitation H ii regions contribute strongly to the [C ii] luminosity in this z = 2.3 ultra-luminous infrared galaxy (ULIRG). The OH feature suggests a maximum wind velocity of 700 km s− 1, which is lower than the expected escape velocity of the host dark matter halo, ≈ 1000 km s− 1. A large fraction of the available molecular gas could thus be converted into stars via a burst protracted by the resulting gas fountain, until an active galactic nucleus (AGN)-driven outflow can eject the remaining gas.