After beating all 8 Kanto Gym Leaders, visit Professor
Oak and he'll open up the path to Silver Cave (or Mt. Silver, both are
named for the same thing).
Go to Viridian and take the Route toward Victory Road. Now it will let
you in where the passages were being blocked by two guys.
From here you can continue out the door to Silver Mountain or go to
Indigo Plateau to fight someone that will be a lot easier than the final
battle in Silver Cave...
Before talking to Oak
After talking to Oak
Indigo Plateau - Rival -
Final Battle
Anytime after fighting your Rival at Mount Moon you can
fight him again at Indigo Plateau right before you go up the stairs to the
Elite Four.
After beating your Rival you can go back to that door to
the Silver Cave area to fight some pretty tough Pokemon.
Sneasel
Crobat
Magneton
Gengar
Alakazam