Where do you catch Groundon.?

Ok so to catch groudon is beat the elite four and after that you will be in you're home again, fly to fortree and go to the weather institute and talk to the guy on the second floor.He will say either there is a heavy drought in route____,or he will say a drought in route____,that is where groudon is(he is in a cave and kyogre is in the underground cave(dive)).
Hope I helped

You have a choice to catch Groudon before or after you beat the Elite Four. First go to the Weather Institute where the dude gave you Castform. Talk to him and he should tell you something about the location of heavy drought or heavy something. When he does tell you, go to that area. A cave should be nearby. Go into it and explore it. Groudon should be there.

Land cave (I think it's in-game name is "Terra Cave"). You need to:
- Beat the Elite Four
- Go to the Weather Institute (Fortree City; go left & down)
- Talk to the man who gave you Castform (he's moved onto a pc).

The man will tell you where an extreme weather condition is. Go there and you will find Kyogre (raining; water cave) or Groudon (drought; land cave) in an entrance that wasn't there earlier in the game. The route changes every-so-often, so you need to be fairly quick.

If the man says where it is raining when you first talk to him, you will have to catch Kyogre first before Groudon. And vice-versa.

Hope I helped! : )

P.s. Sorry I didn't put any fine details into this answer, it would take too long. Another answer I submitted has the details - as far as I can remember.

HAPPY GAMING!!

After you capture or defeat Kyogre, go to the Scientist on the second floor of the Weather Institute and he will tell you the location of a drought (either Route 116, 118, 114, or 115). Go to the location and look for a cave that normally doesn't appear; in the second chamber of that cave should be Groudon.