I believe the slime had been placed in the sewer system several hundred years earlier by worshipers of Gozer. Once the slime had grown in volume and become charged with negative psychokinetic energy, Vigo noticed this and decided to utilize it for his plan. [He shrugs.] Waiting for hundreds of years in slumber did nothing to help him--he should've spent more time studying the effects of the slime.

[He puts his fork down and glances towards Ray.] We did perform a few experiments regarding how it reacted to differing emotional states, both negative and positive. For example, insulting it, regardless of the speaker's tone, would cause it to react, although yelling usually caused it to react more violently.