About 40 meters beneath Tokyo’s Ring Road No. 7 lies a 4.5-kilometer tunnel that receives river water only during heavy rain.
In September 2013, this underground reservoir reached its full capacity of 540,000 cubic meters.
What does “full” actually mean—and what happens to the water afterward?

This video explains why the Kanda River/Ring Road No. 7 Underground Regulating Reservoir was built, how water enters from three rivers, how it is pumped out after a storm, and what its 2013 and 2019 operations reveal about the limits of storing floodwater. It also follows Tokyo’s next step: connecting underground reservoirs and pursuing an underground river toward Tokyo Bay.

CHAPTERS
00:00 The Underground Tunnel That Reached Full Capacity
00:46 Why Tokyo Needed It Underground
01:47 A Giant Reservoir Beneath Ring Road No. 7
02:10 Three Rivers, One Underground Tunnel
02:55 How the Water Falls Underground
03:59 Where the Water Goes After the Rain
04:42 How They Dug 4.5 Kilometers
05:46 Did It Actually Protect the City?
06:57 The Limits of a Giant Reservoir
07:28 The Next Plan: An Underground River

KEY SOURCES
神田川・環状七号線地下調節池パンフレット｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
中小河川整備のストック効果｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
令和元年東日本台風における河川施設の効果｜東京都
 https://www.toshiseibi.metro.tokyo.lg...
平成13年度行政監査報告書（調節池の排水ポンプ・排水時間）｜東京都監査委員
 https://www.kansa.metro.tokyo.lg.jp/d...
神田川上流懇談会 平成26年8月7日議事録｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
神田川・環状七号線地下調節池 流入実績｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
神田川・環七地下調節池の見学に関するお願い｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
環状七号線地下広域調節池の概要｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...
地下河川の取組について｜東京都建設局
 https://www.kensetsu.metro.tokyo.lg.j...

VISUAL CREDITS
Kanda River/Ring Road No. 7 Underground Regulating Reservoir: Joe Mabel, CC BY-SA 3.0, via Wikimedia Commons. Cropped and overlaid with text for presentation.
 https://commons.wikimedia.org/wiki/Fi...
 https://creativecommons.org/licenses/...
Zempukuji River Water Intake Facility: Asanagi, CC0 1.0, via Wikimedia Commons. Cropped and overlaid with text for presentation.
 https://commons.wikimedia.org/wiki/Fi...
Reference tunnel footage: Dominik Zítka / Pexels License
Reference underground-waterway footage: Siarhei Dalivelia / Pexels License

MUSIC
“Long note One” — Kevin MacLeod (incompetech.com), CC BY 4.0
 https://incompetech.com/music/royalty...
“Industrial Revolution” — Kevin MacLeod (incompetech.com), CC BY 4.0
 https://incompetech.com/music/royalty...
“Mesmerize” — Kevin MacLeod (incompetech.com), CC BY 4.0
 https://incompetech.com/music/royalty...
“Infinite Perspective” — Kevin MacLeod (incompetech.com), CC BY 4.0
 https://incompetech.com/music/royalty...
License: https://creativecommons.org/licenses/...

World Blueprint explores the hidden infrastructure that keeps cities and everyday life running.