Are you playing the 13.0 Version of that game? If you are, then either visit the lizardess in her ruins, the one inside the temple in that place, or the witch in her hut inside the swamp and you should have a talk topic about exploring it, do what you need to do for it, 5 glowshrooms for the one in the ruins, don't know what is required for the witch, and you get the item needed to explore it, but have to be equipped before going inside to explore the rooms in there.