Enormous Lyα Nebulae (ELANe) are extraordinarily extended Lyα nebulae discovered around radio-quiet quasars at z~2-3. The ELANe have extremely luminous (L_Lyα > 10^44 erg s^-1) and extended Lyα emission (> 300 pkpc) that reach the inter-galactic filaments. There are two major origins for the extended Lyα emission; fluorescense and scattering. However, the physical origin of the ELANe is unknown. One of the key quantities for distinguishing these origins is the HI optical depth of the extended Lyα components. To constrain the HI optical depth, we have conducted Keck/LRIS spectroscopy of 4 background UV-bright (V = 23.8-24.6) star-forming galaxies whose sightlines penetrate an ELAN; MAMMOTH-1 at z = 2.32. Among the 4 spectra, there is one spectrum of a background galaxy at z=2.52 probing the MAMMOTH-1's bright Lyα nebula component with an impact parameter of b=100 pkpc from the central quasar of MAMMOTH-1. Combining with the Hale/CWI integral field spectroscopy, we measure the HI optical depth of 2.36 (+2.99/-0.67) in the wavelength range of the extended Lyα components, indicating that the HI gas optically thick regime. In the poster, we will present the measurements of our background galaxy spectra, and discuss the physical interpretations of our results.