After defeating Master Belch, you can go to Your Sanctuary at Milky Well.
Go through the cave that Belch was blocking. You will reach a part of Saturn Valley that you could not access before.
Clean Up and have Coffee
Sit in the hot springs near where you entered Saturn Valley after defeating Belch.
Talk to the Mr. Saturn next to the coffee table, and say yes when he asks if you will drink the coffee.
Then go down the ladder, go west and go into the cave there. Talk to the Mr. Saturn on the left to receive some gifts. If you don't have room, go up the ladder to the shop and sell some things, or use the phone to drop things off with Escargot Express. You won't need the fly honey anymore, for example.
Go into the Northeast Cave
Heal up in the hotel. When you're ready, go into the cave north of the hot springs. The Photo Man will take your picture when you walk north of the coffee table.
The monsters in this cave can poison you and mushroomize you, so you might have to go back to Saturn Valley. Just sit in the hot springs to cure your status ailments, including mushroomization and unconsciousness.
Go east through the cave. When you are outside, go south. There is usually a Magic Butterfly in this area. Then go east into the next cave. Open the gift box to get a Coin of Slumber, then go southeast to find the boss of the cave.
Kill the Tough Mobile Sprouts first, then focus your attacks on Trillionage Sprout. These enemies are vulnerable to Fire.
After you defeat Trillionage Sprout and get the Sound Stone melody, go back to Threed and go east to Dusty Dunes Desert.