In an opinion, originally Takamanohara (plain of high heaven) and the Nenokuni were located in the same level as Ashihara no Nakatsukuni (literally, "Central Land of Reed Plains," which refers to the human world), but Takamanohara was moved to upper so the Nenokuni was considered to be located underground.
また、高天原も根の国も元は葦原中国と水平の位置にあったのが、高天原を天上に置いたために根の国は地下にあるとされるようになったとする説もある。