The representation was more in the stylistic representation of the sewers themselves, coupled with the execution of the bright green egg spew, which was the exact technique used for Depths. Again, more of a call back in style and spirit as Legacy posited than a direct, "Here's a Fathom Corp. logo" deal.
This also came from our Unmasking guide after I mentioned the similarities between scenic techniques, so it could have also been blowing smoke (not every guide is infallible)