First, you have to beat the Elite Four. When you do, you can go back to Cerulean cave(When you get to the end of Nugget Bridge, go left, Go south, and surf the waters until you reach the cave) The guy will tell you that you will be able to go in, since it is controlled by the Pokemon league.
mewtwo
If by Meutu you mean Mewtwo, then go to the back of Cerulan cave.
Firered don't have a "solar cave"
You can get Mewtwo in the Cerulean Cave.
You can't
in the land cave.
altering cave,
bartonvilleType your answer here...
Cerulean Cave
mewtwo is in mystery cave :D
The only dark type Pokemon you can get in firered is murkrow which happens to be in lost cave on five island.
Cerulean cave