Route 3 runs off to the east, ending at the entrance to Mt. Moon. There are more than a few Trainers to fight along the way, so it may be necessary to rest up in Pewter City before venturing into the cave.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
The Magikarp Salesman
A Pokémon Center stands near the entrance to Mt. Moon. Inside is a shady man that will offer to sell you a Magikarp for 500. The fish can do nothing but Splash until level 15, but evolves into the powerful Gyarados at level 20. If you have the money to spare, you can have a strong Water-type Pokémon much sooner, but you can soon hook one for free with the right fishing rod.
Collect TM12 (Water Gun) and the Potion to the west, then follow the tunnel around and climb down the nearest ladder.
The presence of a Team Rocket grunt here means that the criminal gang is up to no good here. Grab the HP Up from the ledge and take the ladder back to the first floor.
To the south are two more Trainers. There are also three items lying around, including a Potion, Rare Candy, and an Escape Rope. A Rare Candy raises a Pokémon's level by one, and an Escape Rope warps you to the cave entrance if you need to heal up. Go north and climb down the next ladder.
Beat up the second Rocket grunt, then pick up TM01 (Mega Punch) on the platform. Check the boulder for a hidden Ether and return to the main floor.
Continue along the tunnel to the northwest corner of the cave. Grab the Moon Stone in the corner and take the ladder to the basement.
B2F (outer area)
Follow the winding tunnel to the northwest, where a Super Nerd claims both fossils lying here as his own. Defeat him, and he lets you take your pick of the two. Like with the Old Amber, the scientists on Cinnabar Island can resurrect a prehistoric Pokémon from each fossil. The Helix Fossil will become an Omanyte, while the Dome Fossil will become a Kabuto. Both are Rock/Water Pokémon, and will be at Level 30. Climb the final ladder to reach the exit.