You have to have all the Pokemon including mewtwo
The secret is the Pokemon unown.
Beat the Elite Four, Get Rainbowpass from cello,and tada! You are at seven island
In icefall cave at four island.
It's in Cinnabar island. It's where you get the key to open the Gym #7
you will find it on island 7 when you complete the battle tower
---- the mysterious rumble releases the legendary dog on 7 island.
You need the secret key which is found in Pokemon mansion which is also on cinnibar island.
first go to cinnabar island second loo for the keys at the pokemon mansion
On seven island near tanoby chambers.
Rainbow island doesn't exist.
Go to Pallet Town and Surf down, and you will get to Cinnibar Island. You need to get the secret key from the pokemon mansion on Cinnibar Island to get into the gym.