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
First time: Early in the game, when you first come to Madra, enter the location via the ladder outside the walls of the town (not the ladder inside the walls near the little girl). At a fork leading up and down, it is not necessary to go down because there is a coiled rope near an NPC that merely leads to the area that the ladder entrance near the girl would lead to anyway (and is a dead-end at the moment), so go up. Follow the winding, linear path to the main "underground city" portion, and climb up the ivy immediately visible to your left. Facing an earth pillar from across a gap, cast the Move Psynergy to move it one space left to drop into and fill the gap, letting you proceed forward west. When you get to the "main building" at the upper left portion of the area, you are only able to enter the upper right "cracked-hole" entrance. Inside the main structure, hop right and go down the stairs, and you are lead to the room with a chest containing the Tremor Bit Psynergy item. For now, your purpose in Madra Catacombs is complete, so return to town via either exit or by the Retreat Psynergy.

Second time: After having returned from Osenia later in the game, the ladder entrance outside the town becomes permanently sealed off, but provided you earned the Reveal Psynergy from Air's Rock while in Osenia, the portion of Madra Catacombs accessible from the other entrance inside the town becomes mostly explorable. Enter the ladder near the girl and follow the path to the "front gate" of the underground city area, where a tablet near a sealed double-door commands you to look upon it "with the eyes of truth". Cast Reveal to show a button in the tablet's place and press it to permanently open the main gate.

Now in the underground city, follow the tiled road up and left, but before coming across the main structure to the upper left, enter the gap into the "purple" area where there is a puddle on the ground, and progress beyond that on the ground to get to a chest containing an Apple. Both the earth pillar to the right of the chest and the ivy-overgrown pillar north of the chest can be moved into their respective holes, but the puzzle they would help lead to cannot be accessed at this point in time because you lack the Frost Psynergy to turn the puddle into a pillar and complete a path into the one elevated doorway of the main structure. This is to be saved for the third visit to Madra Catacombs later in the game, so after moving them into their respective holes, go back west and enter the main structure's bottom left doorway.

Much of the interior of the main structure can be freely explored, but the only relevant thing that can be done here for now is to get one chest in particular: in the "main lobby" of the main structure, enter the right doorway, and in the subsequent room, pass by the downstairs stairwell to the left and go up the stairwell above. This linearly leads you to the chest containing a Lucky Medal. Once again, you are done with the dungeon for the time being, so cast Retreat.

Third time: After returning from Lower Gondowan with Piers in your party, all that could not be explored and looted from Madra Catacombs before can now be accessed. Enter Madra Catacombs through the ladder near the girl again, and return to the point where there is a puddle of water on the ground, and use Piers' Frost Psynergy to turn that into a pillar. Now climb up the ivy-overgrown pillar to the upper right and go right to the climbable ivy, but rather than climb down it, walk along the elevated gray stone portion, and hop right across the mound of dirt against the wall (this has filled up what was originally the doorway you first entered the underground city through during the first visit to the catacombs). Run down and left, hop across an earth pillar, hop left to a coiled rope, cast Lash on it and climb across it, slide down one of the two subsequent wall slides but not the other, and hop across the now-frozen ice pillar to the upper left. Hop to the upper left, elevated doorway into the main structure, and enter a small room where a chest is visible on top of a bookshelf that is enclosed by an abyss. Face toward the bookshelf from the left and cast the Tremor Psynergy to drop the chest into the abyss.

Exit out of the main structure and walk along the elevated dirt path from the left part of the main structure to circle the perimeter of the underground city counterclockwise. When you get to a stone section of it near the center/bottom of the area, climb down the ivy to get the chest containing a valuable Mist Potion, then slide down the wall slide to the upper right. Reenter the main structure through its lower left doorway, and enter the double-doors at the top end of the "main lobby" to find that the subsequent room is where the chest from earlier fell down into. It contains the rare item Ruin Key, which can be used right away; return to the main lobby, enter the right doorway, and this time go down the stairwell immediately in front of you to get to a small area where there is a large, wine-colored door. Use the Ruin Key on it to gain access to the Catacombs' ultimate reward, the Moloch summon tablet, which ideally should be your fourth summon tablet. The dungeon is finally complete, so cast Retreat to return to the surface.

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.