The Underground was visually similar to an old sewer system. It was mined out, with the intention of allowing access to the Research Facility entrance. It featured numerous crank and boulder traps, items and secret rooms.
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 double-crosser 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.