Otaki Cavern (大滝鍾乳洞) is one of the largest cave in Tokai region. It is located in Gujo, Gifu prefecture. The entire site is over 7km long and 700m of the area is opened. There is a 30m tall underground waterfalls which explains that stalactites are still actively growing. That underground waterfalls, called Otaki, is the tallest underground waterfalls in Japan.
Tourist Info.
Admission: 1000 yen for adult. junior 800 yen. kid 500 yen. There is a group discount from 15 persons.
Open: 8:30 to 17:00
There is a parking.
Tour takes about 30 min.
Access
0. at Minoshi Station.
1. use Etsuminan Line to GujoHachiman (44 min.)
2. take a taxi to Otakishonyudo (15 min.)
1000 JPY
30 min
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映画好き集まれ！ 
Gujo is famous for it pure water. The limestone layer surrounding the city of Gujo provide its pure water. Otaki Cavern is a huge cave created on one of those limestone layers by water. The stalactite takes 100 years to grow only 1cm. In this cave, you can see its activity of growing.
There are gift shops and restaurants which offers great local foods and products of Gujo.
The official website: http://www13.ocn.ne.jp/%7Eootaki/ootakicavegate.htm