Unlike most other locations, the location of Marine Cave is not set at all; it can be determined by speaking with a man at Hoenn's Weather Institute, who will explain that one of four routes is under abnormal weather conditions. If the player does not reach Marine Cave fast enough, it will temporarily disappear and Terra Cave will appear. However, once Groudon is battled/caught in Terra Cave, or if the player did not arrive there fast enough, Marine Cave will reappear and relocate. It appears only on water routes. In Pokémon Emerald, it is the only place where Kyogre can be caught. It will only appear once the Hoenn Elite Four has been defeated, and it will disappear completely after Kyogre has been either caught or defeated.
Its counterpart is Terra Cave, and neither appears at the same time. The player must defeat, catch, or not visit one Legendary Pokémon before the other cave will appear.