In the northeastern part of the Kibi Plateau, at the top end of Koigakubo Pond and at 550m above sea level, is a hidden spot where Japanese nature remains intact: this is Koigakubo Marsh.

Koigakubo Marsh is designated as a National Natural Treasure. The precious hygrophytes (plants that grow in moist habitats) that naturally grow here show that Japan was once linked to the landmass of continental Asia. Among the 380 types of hygrophytes growing here are Ogura-Sennou, Mikoshi-Giku, Bitchu-Fuuro, Ryu-Kinka.

The marsh is 3.6ha in area and a walking trail around it is 2.4km long. Some 20,000 people visit every year and are fascinated by the mysteriousness of the plant community here. Recently, there has been concern that these precious plants are endangered. The city of Niimi is now cooperating with the Agency for Cultural Affairs to research, protect and manage the marsh and its flora.