Okay first you need to beat the elite four and obtain the national dex then you must go to one island and go to mt ember and defeat two rocket grunts and go inside mt ember from the cave they dug up, go through here using Strength and in here you can find the ruby, deliver the ruby to celio and he will give you a rainbow pass to go to islands 4-7 go to four island and go to the house next to the pokemart and talk to the guy he will give you a sticker for beating the elite four. Now go to icefall cave and find HM waterfall and go up the waterfall and beat the rocket and then go to six island and go to ruin valley and find dotted hole here use Cut on the door to open it and when inside go up then left then right then down and grab the sapphire however it will get stolen. Next go to five island and find and enter rocket warehouse and beat the dude who stole the sapphire then deliver it to celio and finally go to cerulean city and go to cerulean cave by using surf by the water that is near nugget bridge.
By beating the elite 4
Groudon is in Terra Cave, Kyogre is in Marine Cave, and Rayquaza in on Sky Pillar after beating the Elite Four.
Find a specific route or cave to train your Pokemon in and stay there for a long time.
He fled to mistraiton cave if you enter the cave from Route 6 you need to search around for a down staircase cobalion should be in the back of the cave ( NOTE: you should bring a pokemon that knows flash.)
beat the E4, but mewtwos in that cave. articunos in seafoam cave, east of cannivare island(the one with the fire gym)
You have to beat elite 4 to get into Mewtwos cave
By beating the elite 4
You find him in meteor cave after beating the elite four.
you get into the cave only by beating the elite 4 and getting the national dex
NO
I think by beating the Elite Four
When you defeat the elite four now you will go to the cave in Cerulean City.
that cave will open after beating the elite four near route 111
its on routes 105 127or 129 after beating the elite 4.
Western Cave
Cerulean Cave, nor Cerulean City, was not in Pokemon Emerald. In RBY, it opens up after beating the Elite 4.
Complete the following: Beat the elite four and get the national dex and get the ruby and sapphire and deliver them to celio then go to four island and go to the house next to the pokemart and talk to the guy inside he will give you a sticker for beating the elite four now you can officially go in cerulean cave.