M.T Moon
You have to beat the elite 4.
Go through diglett cave.
After you beat the Elite Four, Mewtwo is in the Cerulean Cave. To get to this cave, go across the Nugget Bridge, then surf on that body of water until you get to the cave entrance that was previously blocked off.
In the cave that you used to go to the Pokemon League there was a rock in your way originally. After you beat the Elite Four then go to the end of the cave, and there will be an area that was blocked off. In there you will find Terrakion.
If you are playing Ruby, Groudon is at the end of the cave. But if you are playing Sapphire, Kyogre will be at the end of it.
Go to where the vikings are and go east. You'll see a cave which is blocked by the rocks, use the gunpowder and get in it
On December 10th, Crusoe's Cave experienced a minor landslide due to heavy rainfall in the area. The cave entrance was partially blocked by falling debris, but no injuries were reported. Cleanup efforts were initiated to clear the entrance and assess any structural damage to the cave.
Injun Joe is in the cave to find treasure. He gets trapped inside and eventually dies, as the entrance gets blocked by a cave-in. Tom and Becky manage to escape, but Joe's fate is sealed.
you can catch it in the cave near the ranger school but it is blocked by a rock that needs the move crush.
Diglett's cave is subsequently blocked by a sleepy snorlax. Wake up the Snorlax by tuning to the PokeFlute on your PokeGear radio. You can get the PokeFlute channel by getting the Expn. Card from Lavender Town.
if you mean the one underground with the lock on the door then you have to be a super moshi then do pop goes the googoo mission