"Vienen" is Spanish for 'They Come'.
For a practical joke, the make-up team made sure that the irradiated corpse that Scully examines in the autopsy had a huge prosthetic penis. Gillian Anderson was caught completely unawares by it but found it highly amusing.
Chocolate syrup and molasses were used for the black-oil visual effect.
The only time Agents Mulder and Doggett investigate an X-Files case together (even though technically Mulder was no longer part of the X-Files).
The average X-Files episode took about 18 days to film. This one took 11 because everyone worked on 13-14 hour shifts to facilitate them getting off the oil rig quicker.