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.