You may notice swirling dust in this cave. When you walk into it you will either battle a Drilbur or find a gem. There are a total of 17 gems, one for each Pokemon type. These gems are a one-time use item that increase the attacks of the corresponding type by 50% when held.
As you enter the cave, some people will teleport in front and behind you and rush you over to N. It turns out they are the Shadow Triad, faithful servants of N and Ghetsis. N will leave once he's done talking. Walk north until you are stopped by Bianca. Professor Juniper will show up too and give you the Lucky Egg. It makes a Pokemon gain more experience when it is held. Walk up to the floating rock that Bianca is standing next to and push it to the left. It is magnetized to the bigger rock so it will slam into it. Continue north and you will find an item, but the path to it is blocked by another floating rock. Push the rock to the right and pick up the Parlyz Heal. Take the east stairs to battle a Scientist.
Walk south and move the north floating rock to the right to find a Revive. Go back to where the Parlyz Heal was and take the northern stairs up and walk east. You will see a Heal Ball on the other side of the ledge. Continue east until you battle an Ace Trainer.
There are two floating rocks near the Ace Trainer that need to be moved. The northern one needs to be pushed left and the southern one needs to be pushed down. The will let you walk south along the ledge and then down a set of stairs. Walk east and right as you are about to walk past the floating rock Juniper will stop you and talk about Klink being around for about 100 years. There are two paths here, north and south. Take the north path and battle the Hiker by talking to him.
Take the south path near Juniper and Bianca and you will find a bridge. Right as you walk on it, the Shadow Triad will appear again and rush you across the bridge. They say Team Plasma is waiting for you downstairs. There is a Hyper Potion to the north after the Shadow Triad disappear. Push the floating rock down and battle the Scientist near the stairs.
If you walk south of the stairs and go west, you will find two floating rocks. Push the southern rock left and walk around the big rock to find an Iron, which increases a Pokemon's Defense stat. Go down the stairs.
Follow the path until you see a set of brothers standing by a wall. They are the Nugget Brothers and they will each give you a Nugget for talking to them. A Nugget can be sold for $5000 at a Poke Mart. Keep going west and walk up the stairs.
Don't walk off the ledge because it will lead you to the very beginning of the cave. If you walk around the stairs to the east and head south there is a hidden HP Up underneath the tiny floating rock. Head back to the Nugget Brothers. Walk north and take the west path to battle a Doctor.
Go down the stairs to the west and push the floating rock to the left. Walk southwest in between the two big rocks to find a Thunderstone. Head north and push the second floating rock up to battle a Plasma Grunt.
Walk down the stairs near her to find a Hyper Potion. Walk around the perimeter of the cliff to find a Rare Candy. That's all there is to do on this floor, so walk back up the stairs in the northwestern corner of this area. Walk past the two Plasma Grunts you've already defeated and go up the eastern stairs.
Walk south to find three different paths: west, south, and east. There is a Scientist to the west to battle.