Instructions
Catch the Seagallop Ferry from Vermillion City and travel out to the Sevii Islands.
Go to Six Island, and you will find a door with Braille symbols on it.
Use a Pokemon's Cut move on the door, and it will open and let you into the Dotted Cave.
Jump down the hole in the middle of the first room, and you will land in a room with four holes in it. You will encounter a succession of identical rooms. To get through them, first take the north hole, then the left one, then the right and then the southern hole.
Pick up the sapphire in the final room, and a scientist will come in and steal it from you.
Talk to the scientist, who works for Team Rocket. He will tell you the second password for the Team Rocket Warehouse.