Head back to the village to chat with Gus. The blacksmith says you'll need Orichalcum to make a hammer that can break a giant boulder and explains you can find it in the mine on the far side of the lake. Melanie leaves for the lake while Gus continues to warn about how dangerous the area is. Once Gus realizes that his granddaughter is gone he comments about her spunk, but is worried because Melanie might get hurt. He asks you to head out to the area.
The mine entrance you are looking for will be on the western wall of the map. Once inside, there is a small pathway leading west that you're to walk down. As you go down the path, you and Gus see Melanie walking through the cavern. Gus explains that she is heading in the right direction to get the Orichalcum, but she needs some way of getting across the underground lake. He becomes agitated because this area is prone to cave-ins and rock slides. Suddenly an earthquake triggers, opening a rift in the ground that fills with water. Melanie is trapped on the other side! Gus asks you to head to town and see if anyone has an idea on what to do next, while he stays behind to keep an eye on his granddaughter. You won't be able to have ores processed while Gus is away from his shop.
Head back to town and walk into Naomi and Sally's house. They assure you that Melanie is safe due to the way the Underground Lake cavern is structured. Naomi suggests that a boat can be used to get across to the far side of the lake, as removing the rock slide could make things more unstable. You'll need to collect Hardwood Lumber that will be strong enough to make a boat. The axe you have is only strong enough to chop down Softwood Lumber trees. Rowan suggests you go back and talk to Gus.
At the Underground Lake, Gus is still watching out for Melanie. He tells you that he can make a stronger axe if you bring him 1 Mithril and 3 Iron. He'll head back to his forage in town and wait for you to bring him the supplies. Gus' shop will be open on Sundays while you are collecting the material.
At this point in the game, Mithril Ore is found at the gray mining nodes that are in the same room as the Underground Lake. Your hammer isn't strong enough to hit the blue mining node in this room, and the other mining nodes in the prior room (or in the mine along the north side of the farming area) don't produce Mithril Ore. You'll need to collect 3 Mithril Ore and bring them back to Gus to process into 1 Mithril. Iron can be easily found at the other gray mining nodes. You'll need to find 9 Iron Ore to process into 3 Iron.
Once you have the required materials to upgrade your axe, go back into Gus' shop; you may need to leave and re-enter his shop to trigger this event. Gus makes an attempt to analyze your materials but he is having a difficult time seeing them in his hands. Gus assumes that he is sick and feels terrible for not being able to help his granddaughter.
You leave for a moment and return with Jeanne. She determines that Gus is fine; he just needs a good pair of glasses! Jeanne requests that you bring her 1 Glass, 1 Gold, and 2 Softwood Lumber. You can find Glass Material and Gold Ore at the gray mining nodes, and Softwood Lumber is by chopping the cone-shaped softwood trees. Bring the raw ore (3 of each) to have Gus refine into Glass and Gold, then bring the materials to Jeanne at her house. She crafts some glasses for Gus, which you must take back to Gus so he can try them on. Now that he can see, the blacksmith is energized and plans to make the best axe yet!
After Gus gives you a Master Axe, head out to the farm land and chop down the leafy hardwood trees. Once you have collected the one piece of Hardwood Lumber you need, return to the Underground Lake to hand the wood over to Naomi. You all head over to Doc's laboratory to ask for his help with the boat construction. Doc takes Naomi's lumber and crafts an Insta-Boat. He plans to add enhancements to it but Naomi and Sally insist that you just need a simple raft.
The Insta-Boat can be launched while you are standing at the same level as any water. To continue with the story though, just return to the Underground Lake and the game will automatically play through a scene where you meet up with Melanie. She scolds you for making her wait so long to be rescued, but does thank you, Naomi, and Sally for helping her. Melanie had so much time on her hands that she happened to have found the Orichalcum needed to upgrade your hammer.
Return to Gus' shop to see the happy reunion. Gus gets to work using the Orichalcum to craft a Master Hammer. Now you can hit blue mining nodes and smash rocky ground. Head back out to the farm land and use your upgraded hammer to smash your way down to the entrance of the Underground Kingdom.
The cave to the Underground Kingdom is long, but after the room with many bridges you'll find Gorgon sitting on his lonely throne. The underworld king does not want to talk to a surface dweller. Besides, what goes on above is not his business; it is the surface dwellers that have forgotten to give thanks to the soil beneath their feet! Rowan tells him that it is for the sake of the Harvest Goddess that they ask for his blessing, so Gorgon agrees to help if you find out how his friend the Blue Bird is doing. Rowan suggests that you ask the Harvest Goddess about the bird.
Talk to the goddess when she's available at her spring. She explains that the Blue Bird visits this area on moonlit nights after 8:00 pm. What she means is the bird will only appear on sunny days; cloudy, rainy, snowy, or stormy days will not produce the Blue Bird at the spring. You might also see a dream from the Harvest Goddess when there will soon be a clear night. When the time is right, head to the spring to see the bird, but it gets scared and runs away! Calvin thinks the bird might be injured, so Rowan suggests that you go talk to someone who is familiar with animals.
The next day, head out to talk to Gabriel when he is inside his house. Gabriel assumes that since the bird can fly away so quickly it can't be hurt that bad. Sofia suggests that Gabriel go with you to check up on the Blue Bird.
Head back to the Harvest Goddess Spring on a sunny day after 8pm and you'll meet up with Gabriel. The two of you will find the Blue Bird again, but this time Gabriel's gentle nature convinces the bird to not fly away. After he tends to the bird's injury, it happily flies off and leaves behind a Blue Feather as its way of thanking you.
Take the Blue Feather back down to Gorgon to prove to him that the Blue Bird is okay. Gorgon reveals that a feather gifted by the Blue Bird is much more vibrant than a feather plucked from the bird by force. He can tell that your feather is genuine and promises to cooperate with your task of restoring the Skytrees. Gorgon says the earthquakes are proof that the magic of the sixth Skytree is unstable, so he transports both of you to the Harvest Goddess Spring.
The three deities together cast their spell and awaken the sixth Skytree. With that comes Flint the mining Harvest Sprite and an end to the sudden earthquakes.