Madra Catacombs

Madra Catacombs are a ruinous, deserted underground "city" of sorts located underneath the town of Madra. It is somewhat complex because each of the three times you ordinarily come to Madra (the second time being when returning from Osenia after battling Briggs, and the third time being when returning from Gondowan with Piers), you are able to explore more of the Catacombs than before. The first time you enter you are able to reach the room containing the Tremor Bit in a chest. The second time, if Sheba acquired the Reveal Psynergy from Air's Rock, you're able to explore more of the "city" portion. The third time you can acquire the Ruin Key and obtain the location's biggest secret, the Moloch Summon Tablet.

Walkthrough
Head into the catacombs via the entrance down the ladder by the little girl (not the one outside the walls). Follow the path around to the left, and in the next area, climb down. The tablet to the left of the door is actually a switch, so use Reveal and push the hidden button to open the doors. Go left inside, and north to a puddle. Use Frost on it. Go northeast to the viney pillar and push it into its hole. Head down and around the wall to get a chest containing an Apple. Go right to the pillar, and Move it across. Go back to the vine pillar, climb it, and jump across to the top of the ruin walls. Head across and around the walls, to the ledge formed by the slumping earth. Follow the ledge around to the pillar you moved before.

Across here, use Lash on the rope. Drop down the first groove, hop across the frozen pillar and onto the top of the building, then head through the door. Inside go through the left door (you can't actually go anywhere else). Use Tremor in here to get the chest off the top of the bookcase. Head back out. Follow the ledge to the left all the way down and climb down the vines at the end to get a Mist Potion. Climb back up and use the groove to get back down to the floor of the ruins. Head north to the door below the one you just entered.

Inside, go through the double doors to the north. In this room, get the Ruin Key from the chest (this is the chest that was on the bookcase earlier). The stairs up to the right lead nowhere, as does the left door, so head through the right-hand door. Head north and up the stairs. Follow this path to get a Lucky Medal from a chest. Now head back and go down the stairs you passed in the familiar room.

The huge door down here can be opened with the Ruin Key. When the torches have lit up, head up to the combo tablet. You can now summon Moloch! He requires 2 Mercury and 1 Jupiter on standby. Now, Retreat from the ruins. You can use the Apple on whoever has the lowest Attack.

Collectibles
When Madra can first be entered early on in the game after exiting Dehkan Plateau, some of the Catacombs can be explored, but not much. The following can be acquired at this point:

Tremor Bit: In a chest in a room in the main "structure".

More of the Catacombs can be explored once the party is in possession of the Reveal Psynergy found in Air's Rock. The treasures obtainable now are:

Apple: In a chest in the "exterior" area below an elevated coiled rope. Lucky Medal: In a chest in a broken hallway in the main "structure".

But once you gain Piers, who allows the party usage of the Frost Psynergy, the rest of the catacombs' secrets can be looted:

Moloch: A summon tablet that can be viewed as the dungeon's final treasure. Reachable only by using the Ruin Key on a large dark door.

Mist Potion: In a chest in the bottom of the "exterior", reached by navigating one's way to its area on top of elevated portions of wall and such, requiring the Lash and Frost Psynergies to get to it. Ruin Key: In a chest atop a bookcase; to get to it, cast Tremor, which will cause it to drop down a pit into a different room below, where it can be picked up.