Stalactite Cave

Items found: Gold Nugget, Iron GWhip

After Bart and Fei introduce themselves and a scene between Citan and Sigurd occurs, you can move around with the 2 gears. Head South, then West until you find a big rock and you'll shove it off. Follow the passageway and head South to get a Gold Nugget, then West to another entrance. Cross the long corridor, then cross the bridge on the next area. When you reach the standing Gear, head West, then North and jump 3 platforms to get an Iron GWhip. Go back to the Standing Gear and head SouthWest to the next area. After jumping a few platforms, use the Memory Cube and get outside the Gears to enter the cave.

After some scenes with Bal, talk to him again to upgrade your gears (I recommend leveling up until you can get all the upgrades). Go back to the Standing Gear area and head to the platform where you got the Iron GWhip and turn off the sensor on the second platform. Now go back to Bal's Cave Area and head West, then jump all the way North until you find a passage to heading East. Notice that in the next area you'll be above the Standing Gear and cross the bridge. Follow the long corridor and head West on the next area to deactivate the second sensor. Now go back all the way to Bal and talk to him. He'll open the sand barrier and after a scene you can continue your way. Just follow the only path (get on the gondola to activate the elevator) until Calamity shows up.

Calamity may give you some trouble if you didn't upgrade (He can take over 400 damage), so activate booster and use Skills like Reppu (Triangle, Square) and Beat Serpent (Triangle, Square). It may take a while, but eventually he'll fall. Exit the area a scene in the desert and the reunion with Citan, you'll be moving around the Yggdrasil.