Cinnabar Volcano

Cinnabar Volcano connects up from Route 21, a route from Saffron City. The volcano is a large mountain, which takes quite a while to travel upwards upon. In place of the town located on Cinnabar Island, the original game's location, is a city on top of large rock-like supports, creating a Floating City.

Obtaining Lugia
You can obtain Lugia by doing the following:

Talk to the NPC in black in Cinnabar Volcano's PokéCenter.

He will also give you a riddle/list as follows:

"Lazy bear. Dragonfly. Doctor. Fire bird. Magnets..."

Which means:

"Snorlax. Vibrava. Chansey. Moltres. Magneton."

Have these 5 Pokémon and an empty spot in your party and he will give you Lugia. (Possibly Shiny)

Obtaining Rotom
Rotom could be obtained in a 2015 event in which you could find it if you had a -type Pokémon and an -type Pokémon in your party. Now, it is only obtainable through trade or Pokémon Roulette.

Cinnabar Gym
The Cinnabar Volcano Gym Leader is Blaine. He specializes in -type Pokémon, and gives the Volcano Badge and TM Overheat when defeated. Once you have defeated him, you go back to Viridian City to battle the 8th gym leader.

Unlike other Gyms in-game, Cinnabar Volcano is the only location to not have a building designated for its Gym. Instead, players have to battle Blaine on top of the volcano.

Illegal Pokemon Shop
One NPC will offer Pokémon to the player for Pokédollars. To shop with her, you must have at least 1 empty slot in your Party.

Italics = Can be found in the wild.

NPC Scientist
On top of the volcano, an NPC is located, if you talk to him he will give you information about Mega Evolution and will give you a free Pidgeotite; a Mega Stone used for mega evolving Pidgeot.

'Hunch' Areas
Hunch areas are Routes/places that have Legendary Pokémon roaming in the patches. Talk to the NPC in the last house and he will say, "I just have a feeling that ______ has a rare Pokémon! These hunches change frequently, so you should come back in a couple of minutes!" These hunches last for 5 or 10 minutes depending on which Route/City.

Route 1: 10 minutes

Route 3: 10 minutes

Route 5: 10 minutes

Outside Mausoleum of Origins: 10 minutes

Pokemon Tower: 10 minutes

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