The Underground was located within the grounds of the Spencer Mansion's Courtyard. In the original version, it led to the secret entrance of the Arklay Laboratory. In the remake, it led to an underground tunnel before coming out in the cabin. It was reached through a waterfall in the courtyard.
The Underground was visually similar to an old sewer system. It was mined out, with the intention of allowing access to the Laboratory entrance. It featured numerous crank and boulder traps, items and secret rooms.
The Underground featured a number of different enemies:
It was in the Underground that S.T.A.R.S Bravo Team leader Enrico Marini was found injured. He informed Chris Redfield that there was a traitor within S.T.A.R.S (Albert Wesker). Before announcing who the traitor was, Wesker assassinated Enrico from within the shadows, and ran away before Chris could notice him.