Secret of Mana: The Underground Palace

The Underground Palace is a Quest in Secret of Mana (SOM).

Description

The party obtains the power of the water elemental, Undine. Sage Luka tells them to return to Gaia's Navel and the Dwarf Village. From there, they must continue down into the Underground Palace.

With Undine's magic added to the party, it is time to make your way back to Gaia's Navel.
Exit out of Undine's Cave and head West.
Turn South at the entrance of the Water Palace and then head West to reach the teleporter to the Haunted Forest.
Make your way out of the Haunted Forest and over to Gaia's Navel.
Spiral inward and use Watts's "secret" entrance into the Dwarf Village.
Once inside, rest up at the Inn (15 GP) to restore your MP and HP before beginning the Underground Palace.
Head Southward from the Inn to reach the entrance to the Underground Palace, it sits in the wall immediately opposite the entrance to Dwarf Village from Gaia's Navel.
Once inside, you will need to an Undine spell on the orb sitting on the pedestal.
A quick cutscene will run and then you will see the lava has cooled, allowing the party to reach the Underground Palace itself.

Just across from where you enter the Underground Palace you will encounter 2 Goblins.
Defeat them and then go through the double-doors in front of you.
Here, in front of you, is another Goblin in front of a large brazier blocking stairs upwards.
Go left after you have defeated this Goblin.
Face and defeat the Chess Knight that is waiting on either side.

Head South from there, down the stairs.
At the bottom of the stairs to you will find a Green Drop waiting for the party.
Just a bit farther in is a Blat.
Defeat both these foes and go to the end of the walkway.
Step on the floor switch.
A sound will be heard, so head back up the stairs to the main chamber you were just in.

At the top of the stairs you will find a pair of switches.
Step on the green floor switch (left-hand switch) to have the bridge extend.
Go North across the bridge and up the stairs there.
Head over to the West and you will encounter another Goblin.
Defeat it and turn South, heading down through the next set of doors there.
Go over to the West on the far side of the doors, defeating the Goblin.
Equip the lead character with the Whip to get across the gap in front of you.
Immediately, on the far side of the gap, is a Green Drop.
Defeat it (and its possible spawn) and then head Northward.
This will lead you to encounter a Ma Goblin and a Goblin.
Go through the archway to the North.
Before the next set of stairs you will encounter another Goblin.
Defeat it and head on up the stairs.
In the next room, behind the large brazier in the middle is another Goblin.
Go through the doors to the North.
Just through the doors is a Goblin, and to the Northwest is a pair of Chess Knights.
To progress forward, go through the Northwestern doorway.
Fight the Goblin in the hallway on the other side of the door.
Go on up the stairs to reach the next room.
In this room, the party will meet Gnome. Who will immediately start arguing with Popoi.
A large rumble is heard, then you can head North into the next room and take on the boss, Fire Gigas.

Fire Gigas will likely begin the fight be turning into a mass of fireballs.
Immediately run away from them, going around to the opposite side of the room.
Throughout the fight, watch your party's health. Immediately use Cure Water when most of the party is at 50% or less health.
Wait until the Gigas reforms its down.
Use your Undine magic then to cast Ice Saber on the party with Primm and Freeze on the Fire Gigas.
With rapid casting of Lv. 1 Freeze, this will be a very easy fight.
Mix in weapon and charged attacks to make short work of this opponent.
Just be sure to run anytime it becomes a mass of fireballs, it is invulnerable during this time.

With Fire Gigas defeated, head to the North and up the stairs there.
Just go straight forward from there and toward the stairs to the seed.
Gnome will reappear and, eventually, offer his powers to Popoi and Primm.
After the conversation, go up the pedastal's stairs and interact with the Mana Seed there.
This will link the seed to the sword and then Popoi will touch the seed and regain their memories!