There is no secret. Its just there to make you think.
Nothing.
the secret key is in the Pokemon mansion - Celadon city basement
Nope.
To get surf you got to get to the secret house in safari zone
In generation 3, the only games that include "secret bases" are Pokémon Ruby, Sapphire, and Emerald. The LeafGreen and FireRed, Kanto reboots, unfortunately did not include this particular feature.
If you mean the 9 islands its in Firered or Leafgreen.
you can't mix records and if you could you can get there secret base and other stuff but you can't mix records with firered and emerald.
The secret key opens the cinnibar island gym.It helps at the rocket hideout or the Silph Co. I forgot
go to the eighth floor and beet the trainer
Go inside and talk to the guy inside he gives you the HM Surf.
The secret house in the safari zone by the golden teeth
It serves one purpose and that is to unlock the Cinnibar Island gym.