After defeating Electro Specter and falling from Lumine Hall, you find a huge underground world.
Teleport if Needed
The Lost Underworld has been added to your Teleport list, so feel free to teleport away and come back if you need to.
Don't try calling Escargo Express or Mach Pizza or using your For Sale sign while you are here. They will not be able to reach this location.
Go northwest from where you start. There is a small hole just west of a lake. Sometimes the ground will shake, then a geyser will shoot out of that hole. If you stand on the hole before the ground shakes, you will stand on the geyser when it comes out, and it will heal your HP and PP. You will find a couple of other geysers in the Lost Underworld besides this one.
Go west from there, then go northwest when you can. You will see a fenced-in area, with a rock wall to the left. Go west of that rock wall to find a gift box that contains a Brain Food Lunch. Then enter the fenced area. You can use the bird phone there to call Mom and Dad. But as mentioned before, Escargo Express and Mach Pizza can't reach you here.
Now go east, and make your way to the southeast corner, where you will find a gift box that contains a Horn of Life.
Go north from there and you will find a gift box in the northeast area that contains a Sea Pendant.
Then go south until you can go west past the rock wall. Keep going west until you have gone past a lake to the north. Go northwest past the left side of the lake, and you will find a gift box to the north, to the left of a river. It contains a Guts Capsule.
Tenda Underworld Village
Go south from there, then go west when you can. You will find a fenced-in area. Go to the door of the fence, which is in the middle of the south wall. As you approach, if you have the Tendakraut with you, a Tenda will open the door for you.
Talk to the talking rock to learn about the Your Sanctuary locations. The talking rock also heals you for free.
One of the Tenda is running an item shop, so be sure to buy upgraded equipment from there.
There is also a Tenda who can give you money from your bank account, but he charges a 100% fee. So for example, if he gives you $100 cash, you will lose $200 from your bank account. (If you are wondering about that Tenda's weird name, read Tomato's explanation of the name Ay-go Stikke.)
There is a red geyser north of the Tenda area. The red geyser heals status ailments.
Go west from the Tenda area and you will see a cave door. There is nothing to do in there right now, but you can go in to hear some weird music and see something strange.
Back in the underworld, go south until you can't go farther south, then go northeast to find a gift box that contains a Cloak of Kings for Poo.
Go south from there, and you eventually find a blue geyser.
To the left of that geyser, there is a fenced area with a bird phone inside.
Go west from there to find a cave door. Go through that door to reach the Fire Spring cave.