Ascension Island is accessed through Lonely Cave, it's the default map for Feral Heart. You go on top of the den to get there. South Pole is also accessed through this default map. Through The South Pole, you can reach Atlantis, I believe it's in the top right hand corner of the map? Maybe left. Last Cave is reached through Atlantis, under water. Sky's Rim is accessed through Ascension Island once you reach the top.
You have to be playing the regular Feral Heart, not Plus, to get to all these maps.