Do you ever wonder what Groudon in pokemon was based off of? A godzilla maybe, or maybe something larger, and more realistic. Sakurajima volcano in Japan probably houses the answer , very close to the legendary
suicide forest, it quite possibly harbors a groudon.
Now how can we get it to come out? Look:
groudon found
That lightning is the life force given by arceus to make groudon come out of the ground. When it comes out, people will have to go find a kyogre in the pacific ocean, although it too might end up attacking these poor people.