in gold i think it is to infiltrate the radio tower where team rocket have taken over in the town with the 3rd gym at least i think
you use it to open the door to the basement!
oh hai
Catch the bat on the roof and use the key to get in the basement.
use the basement key in goldenrod city. the key is in goldenrod.
you find the basement key in the underground path
The basement key is used in the Goldenrod underground. You get it while rescuing the Radio Tower from Team Rocket - specifically, after defeating a Rocket Executive disguised as the director.
The basement key can be found in the room with projector. The projector shows animals that correspond with pictures of animals in the room that can be pressed. Press them in the correct order to reveal the basement key.
the key is in the basement
After you get the basement key, you go into the tunnel that is under Goldenrod City and you go to the end of the branching off hallway. There, there is a locked door, where you use the basement key and the door opens, if you get all the way to the end of the tunnel behind the door, you find the director there.
In Pokémon Brown, the key card is typically found in the basement of the Pokémon Center in the city of Oaken. You'll need to navigate through the building and defeat certain trainers to access the basement area. Once you reach the basement, look for the key card on a table or in a specific location. After acquiring it, you can use it to access locked areas in the game.
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I don't know if it's the same as in Sapphire but I think you have to go to Mauville City and get the Basement Key off the gym leader there, Wattson, then go down to wear the man is fishing. Use surf there and go right. You will come across 2 islands one that has a tree and a sign saying something like 'Team Aqua was here, but scribbled oover was, Team Magma Rules!' Surf some more and on the next island there is a metal door, and that's where you use the basement key.