This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
You can start by searching for MAGI in the city sewers.
You can't enter the sewers from inside Venus's City, but you can from
the outside. There should be another way in somewhere around the
Outcasts' Village. (Look behind the city walls.)
After you get all the MAGI in the sewers, talk to Venus & then Flora in
Venus's City to make the volcano rise. Go into the volcano to find more