Ancient Ruins
The Ancient Ruins was broken between the time Hobson left and the time Bea discovered it due to someone from the future (who had heard Hobson or Bea's stories, perhaps) coming in very shortly after Hobson had left and breaking it.Rainbow Island was inhabited by the same civilisation who built the Ancient Ruins
Rainbow Island is filled with similar structures to the Ancient Ruins, such as the walls made of multiple blocks of stone with circles in the middle, the plort-activated slime statues, and big teleporters.