This swamp was originally called "Takeurakei" or “Valley Behind the Bamboo” because of its location behind a bamboo forest. Benkei Bridge is also known as "Akahashi" due to its bright red color. Since 2003, Chinzanso has been holding the "Firefly Larvae Release Ceremony" with elementary school students from around Bunkyo Ward. At the end of May, they invite students and parents to watch the fireflies become adults and enjoy the sight of them flying around the garden.

“Ho ho, fireflies come! Over there the water is bitter, over here the water is sweet!”

This famous children's song has been sung since the Edo period during the firefly catching season. Catching fireflies was a summer tradition of the people of Edo, who are said to have hung lanterns over the river and ran around with fans trying to catch fireflies. It's hard to believe now, but even in Tokyo, fireflies were around until the 1950’s. The magical scene is brought back to life every year from mid-May here in the Chinzanso garden.