keep doing the adventure then when kyore and groudon are fighting you will be able to go in it

Wats Mt. Origins?

mount origin is where you find that wallace dude and will ask you where's rayquaza and tell him it's in the sky tower then get out of the cave. once you went to sky tower and woke rayquaza up and setteled the two rival pokemons(groudon and kyogre) you can not go back into that cave. that kinda sux coz in ruby and sapphire version it doesnt do that.

its called (cave of oregin)