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tokens: have a Ly α luminosity of . It was the first SMG detected in CO, revealing it to contain a massive reservoir of molecular gas ( M ⊙ , Frayer et al. 1998 ). This gas reservoir was later shown to cover a large volume and contain multiple, merging galaxies (Ivison et al. 2010 ), one of which is a dusty, broad-absorption-line (BAL) quasar (Villar-Martín et al. 1999a , 1999b ; Vernet Cimatti 2001 ; Frayer et al. 2018 , e.g., Genzel et al. 2003 ). The existence of a large reservoir of diffuse molecular gas has been recently inferred by observations of the CH + cation (E. Falgarone et al. 2019, in preparation), following the detections against several other SMGs (Falgarone et al. 2017 ). Inspired by these earlier results, we observed the system with KCWI tuned to the Ly α nebula and directed to cover it and its neighboring galaxies and quasar. To our surprise, the data reveal a new, luminous Ly α source—the primary focus of this paper, which is structured as follows. We describe our KCWI observation and data reduction process in Section 2 . In Section 3 , we present the detection of the ≳100 kpc Ly α nebula powered by this system and the discovery of a new dark cloud illuminated by the nearby BAL QSO. Finally, we discuss the Ly α nebula and the physical interpretations of the newly discovered dark galaxy in Section 4 , and give a brief summary in Section 5 . Throughout this paper, we assume a flat cosmological model with and , which implies the physical scale is ≈7.85 kpc per arcsec at the redshift of SMM J02399−0136. We follow the naming convention presented in Ivison et al. ( 2010 ): L1 marks the BAL quasar; L2 is a low-surface-brightness companion to the east of L1; L2SW is a massive, extremely luminous and dust-obscured starburst galaxy, east–south–east of L1 and south–west of L2; L1N is a faint, compact northern component, visible in post-SM2 HST imaging. 2. Observations and Data Reduction 2.1. KCWI Observations The KCWI observations of SMM J02399 were carried out on UT 2017 October 21 (seeing ∼15) using KCWI on the Keck II telescope of the W. M. Keck Observatory in Hawaii. We used the BM1 grating and the medium slicer (slice width ∼07), which yields an IFU datacube with FoV of 20′′ × 168 (pixel scale of 03 × 07) centered on J023951.88−013558.0, the quasar optical position given by Ivison et al. ( 1998 ). The grating was tilted to give a central wavelength of 4620 Å and provides a spectral resolution, R ≈ 4000. The good wavelength coverage is ∼4230–5010 Å. The total on-source exposure time is four 10-minute exposures, each dithered by ∼06. For sky observations, we used an “offset-target-field” to construct the sky datacube. The offset-target has a different redshift and is located ∼2 degrees from SMM J02399. It is a compact point source at the Ly α wavelength of SMM J02399. To convert the spectral images and calibration frames (arcs, flats, bias) to a calibrated datacube, we used the IDL-based KCWI data reduction pipeline. 11 11 Available at