Where did u find it cuz it depends were...did u find it in magma hideout?

If it is in Emerald, then you will have to beat Wallace in the Pokemon League first. Afterward, go to the Weather Institute on Route 119, and talk with the scientist on the second floor. He will tell you that there is flood/drought activity on *enter whatever Route here* and that it is an interesting weather phenomenon.
If you go to whatever Route he mentions, you will find that there is an area that wasn't there before (cave entrance to Terra Cave for Groudon, and diving area in water to Marine Cavern for Kyogre... I think that's the name of the place). Once you go in, you cannot go out again without the caves disappearing forever, so enter prepared. Each of them is at Lv. 70. Once you defeat/capture one, talk to the scientist again, he will tell you the location of the other. The locations tend to vary, so if you save, and take out both of them, and turn off without saving those defeats, the scientist may tell you different locations.

i found it in aqua hideout

scrap that^ i found it in seafloor cavern (where the sub is)

okay all of ur info
1:magma hideout is in jagged pass after u get magma emblem
2:groudon is at the end when u beat maxie
3:u go 2 lilycove city again 2 go 2 aqua hideout
4:when u beat every1 there go 2 seafloor cavern near sootopolis city (u need dive)
5:kyogre is @ the end u scare it off and battle archie

i forgot 2 put up there 4 ur info by the WEEEguy
from;the WEEEguy

if ur in the seafloor cavern then u probably did sumthing wrong if ur in marine cave then all u hav 2 do is go up to the middle of the pool thing and i think press a

hi deoxeon

hi WEE guy