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Marine Cave (Japanese: うみのどうくつ Marine Cave) is an area in Pokémon Ranger: Shadows of Almia.
Marine Cave is inaccessible until the mission "Investigate the Marine Cave!", when the tide lowers and lets the player enter the cave. In the mission, Marine Cave is under the effect of an unknown machine when the player and Crawford first investigate a mysterious noise. This machine, later revealed to be a red Gigaremo unit, hypnotized the Pokémon inside the cave to prevent them from be captured as friend Pokémon. The player must capture a Gastrodon and use its Field Move, Soak 2, to destroy the Gigaremo machine. At this point the mission is completed. Crawford will try to carry the machine back, only to realize how heavy it is. He then calls upon Big Bertha to lift it to the Ranger Base in Vientown.
Marine Cave's layout is relatively simple. It has three rooms, all of which appear to be on the same floor.
From the beach entrance, there is a giant room that is being controlled by a Gigaremo unit until the mission "Investigate the Marine Cave!" is cleared. In the northwest corner of the room is the entrance to another room.
Again there is an entrance to another room in the northwest corner. It is blocked off by a rock which requires Crush 2 to access. However, Nosepass is able to help out.
Gastrodon can be found here. Gastrodon is required for the Marine Cave's Mission to be cleared.