忍野八海
忍野八海(おしのはっかい)は、山梨県南都留郡忍野村にある8か所の湧泉群。富士山の伏流水及び杓子山から石割山にかけての山麓からの伏流水を水源とし、新富士火山の透水層の地下水及び古富士山の不透水層部より下方の透水層のうち水圧の高い地下水が湧きだしたものとされている。忍野八海からの湧水は新名庄川に流れて、山中湖を水源とする桂川へと合流する。
国指定の天然記念物、名水百選、県の新富嶽百景にも選定されている。2013年には「富士山-信仰の対象と芸術の源泉」の構成資産の一部として世界文化遺産に登録された。
かつては忍野村域にあった忍野湖が干上がって盆地になり、富士山や近くの火山山麓の伏流水を水源とする湧水の出口が池として残ったのが忍野八海である。平地となったこの一帯は米の産地となり、湧泉の水は飲料水や農業用水として活用された。
「八海」の名は、富士講の人々が富士登山の際に行った8つの湧泉を巡礼する八海めぐりからきており(多数ある湧泉のうち8つに限ったのは、8を尊ぶ仏教的思想に基づく)、富士講では忍野八海のことを、元(小)八海、内八海、根元八海、富士外八海などと呼んでいる。古くから富士修験の霊場とされ、行者たちは富士登拝の前に池でみそぎを行なったとされるが、1843年に各池に守護神の「八大竜王」が祀られ、出口池を一番霊場、菖蒲池を八番霊場とする巡礼路が整備された。
忍野八海はその特異な自然現象から、訪れる観光客も多く、富士山周辺でも人気の観光地として知られる。周囲は観光用の商店や施設などが建ち並び観光地化が進んでいる。
地元の旧家に残る古文書「宮下文書」の伝説では、もともと当地一帯には宇津湖(宇宙湖とも)と呼ばれる巨大湖があったが、800年から802年にかけて起こった富士山の延暦大噴火で流出した鷹丸尾溶岩により宇津湖が分断され、山中湖と忍野湖に分かれたと伝えられている。しかし標高差の地形的観点および山梨県環境科学研究所のボーリング調査により、宇津湖の存在は否定されている。
1934年 - 国の天然記念物に指定された。
1985年 - 環境庁(現・環境省)の名水百選に選定された。
1987年 - テレビ朝日系の番組撮影で湧池の洞内に潜水した潜水士2名がリールを持たずに入り死亡し、遺体回収に2週間を要した。
1994年 - 山梨県の新富嶽百景に選定された。
2013年6月22日 - 「富士山-信仰の対象と芸術の源泉」の構成資産の一部として世界文化遺産に登録された。
忍野八海の観光化は、茅葺屋根と富士山を背景とした田園風景を売りに1960年から始まった。1965年ごろから観光地として注目されるようになり、1985年の名水百選選定で観光地化が進んだ。
忍野八海地区からは見える富士を忍野富士と呼ぶ。冬季には池より朝霧が立ち込めることもある。最も深い「湧池」を中心にして7つの池を巡る観光客が多く、一つだけ離れた位置にある「出口池」までを巡る者は少ない。また、あまり目立たない「鏡池」「菖蒲池」などを省く観光客もおり、そちらも比較的に人出が少ない。歴史は浅いが毎年8月8日には守護神の八大竜王を祀る「八海祭り」が催される。
忍野八海はその名の通り、以下の8つの池のことを意味する。人工の池については節#人工の池を参照。
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