DiMA's cache is a small island just northwest of Rayburn Point on the western edge of the map. It is where DiMA buried the kill switch for the Island's wind farm, which must be retrieved for the quest Cleansing the Land. A mound of "disturbed earth" can be activated, unearthing a tool case containing the kill switch code.
There is an island a short way north of DiMA's cache which is just barely outside the map boundary. Two mirelurk queens can spawn there.