The room in the frontal "exit" portion of Venus Lighthouse has that room with the big statue and the glowing circles on the floor, and talking to the statue will cause the circles on the floor to connect differently. This represents a different path that is now open in the Tunnel Ruins dungeon area for the player to go through, and you go into Babi Lighthouse, then go through the Tunnel Ruins through the path you selected. Both paths in the Tunnel Ruins lead you to the actual dungeon of the Venus Lighthouse (and each path contains its own unique armor to collect, Asura's armor for the right path and Oracle's Robe for the left path), and you know you're in the dungeon when the music is new, a Psynergy is on the ground, and a tree pattern is on the ground.