How do you get into the Cerulean Cave in Pokemon?

Answer 1

fire red leaf greenits pretty simple.

you just have to do the Pokemon league and the go to cerulian the go up past the five trainers and surf till you can see a man and he says this is cerulian cave blah blah blah n then he lets you in i think that's it
You first must defeat the 8 gym leaders and the elite 4. Then you will be able to go into the cave and catch mewtwo. Try going on Google and look for a walkthrough of the cave.
Become Pokemon champion, receive the national dex, deliver the ruby and sapphire to celio on one island. After doing all of that you can enter cerulean cave.
To get to that cave you must beat the elite four at least one time and have the national dex by catching 60 Pokemon then talk to prof oak After you have done that, you need to go to island 1 and talk to Celio. He will ask for the ruby and sapphire, give them to him.

After that, you will have access to the cave.