The Deep Darkness is a large, scary swamp that in some areas is pitch black. When you arrive, you'll be on a dry part of land. A tropical bird with a telephone handset on its head is nearby. Talk to the bird to use it as a phone. It's free. And, if you don't have the Hawk Eye, call Escargo Express to get it.
Also, there is a monkey standing in the trees south of the tropical bird phone. Talk to it to heal up for free.
Just east of the bird phone and the monkey hotel is a shallow watery area followed by a deep watery area. The deep water is dangerous. You have to walk underwater, which is slow, and it also slowly reduces your HP. Be careful to keep an eye on your health.
Once you get into the deep water, you can put the piggy nose on and use it, because a Magic Truffle is near. Be sure to use the piggy nose in the Deep Darkness, because this is the only place where you can use it to find Magic Truffles.
Head east through the deep water to reach some shallow water and then some dry land. An arms dealer will be walking around nearby. You can buy some good stuff from him. There is also a Magic Truffle near where he walks around.
Go north from the arms dealer to find several things. A monkey will ask you to teach it how to teleport, and if you do, you get the Monkey's Love I think it immobilizes the enemy for a turn. Also in this area is a businessman who sells some things and will work with the doctor nearby if you need to cure a status problem. The businessman will agree to pay for stuff for you. Don't choose Cash after that, or you'll have to pay him. If you never pay him, it adds something to the ending of the game.
When you're ready, go south from the arms dealer. After going through some deep water, you'll get to the totally dark part of the Deep Darkness. Use the Hawk Eye and the darkness will become bright. Be sure to get the banana from the gift box around here.
Next, go south. Talk to the snorkel thing to find a guy who will act as an ATM, but he'll charge you a fee of the same amount that you withdraw. So it costs four hundred dollars to withdraw four hundred dollars. You can Teleport to a free ATM if you really need money. The snorkel man isn't that far from the beginning of the Deep Darkness.
The path is pretty straightforward from here until a tree in deep water with a log to the left of it. Continue left, get the beef jerky from the gift box, and use the piggy nose to find the nearby Magic Truffle. From there, go left to find an IQ capsule.
After that, go south from where you found the Beef Jerky. Follow the straightforward path here and you'll reach the helicopter that Pokey stole from Monotoli. The helicopter is broken beyond repair, and Pokey is nowhere in sight. North of the broken helicopter is a Cup of Lifenoodles.
Continue left from there. You'll have to trudge through a long expanse of deep water, so keep an eye on your health. Once you reach the shallow water at the other side, use the piggy nose to find a nearby Magic Truffle.
Go left from there, and continue left to find a Souvenir Coin. Then return to the path that was going north. Go up that northbound path. When you get into the deep water, head north and slightly east to reach shallow water. Keep going north from it to get a Rock Candy.
Go back to the deep water and go east. You'll run into Master Barf, which is a reincarnation of Master Belch. Find out how to defeat Master Barf.
Once Barf is gone, Poo will be back with you, and you'll get a Casey Bat. DON'T USE CASEY BAT!! It's a terrible weapon that almost always misses. I equipped it when I first played through EarthBound, and I got really frustrated and confused with how terribly Ness was doing, until I realized that it was the stupid bat.
From here, just go north. When you see shallow water, start going northwest. Once you're in the western piece of shallow water, use your piggy nose to find a Magic Truffle near the trees north of the two flowers.
From there, head west. You'll reach the cave entrance to the Tenda Village.