Golden Sun: The Lost Age/Madra Catacombs

Go down out of the ladder room and go around to the top door. In the next room, go up and climb down the ladder. Go out the bottom of this room. You'll be in the ruins of whoever the ancestors of the Madrans are. Anyway, go in the hole in the wall next to the door you came through and climb up the vines. Use Move on the pillar to drag it into the hole so you can walk across. Go down the vines and enter the building. There's only one path you can take for the mean time, so follow it down the stairs and keep going until you reach a chest. Inside is the Tremor Bit, which you should equip on Felix. You can't use this power yet, so Retreat from Madra Catacombs and exit onto the world map. Don't go to Madra Drawbridge (the bridge east of Madra) just yet though - instead, go across the bridge over the river to the west of Madra.

Iron, the Venus Djinni
There's a large forest here with a green patch of grass nearby; wander around in here. Eventually, Iron will show up in a random battle. Defeat it like you would any other foe, but just remember that Sheba may take a bit of a pounding in this battle since it is a Venus Djinni and is very fond of casting things like Growth and Spire. If it runs, just keep wandering around until it reappears. Once you've defeated it, it'll join your party (make sure to Set it and put it on Felix) and go east of Madra until you reach the bridge.