What's Really Under Tokyo (Mapped)

Under a busy Tokyo ring road runs a flood tunnel that can hold as much water as 216 Olympic swimming pools. On the city government's count, about 124 km2 of eastern Tokyo, about a fifth of the 23 wards, lies below the usual high-tide line, and at the worst point the ground has sunk about 4.5 m, measured since 1918. On one real map of Tokyo's 23 wards, we follow the low east from the 1910 flood to the pumping that sank it and the walls that keep the water out, then go west to crowded streets where small rivers are hard to widen, the Ring Road 7 tunnel, and the two typhoons that tested it. By the end, you'll know what's really under Tokyo (tunnels built to swallow floodwater) and why this city fights water from two directions: rain from above, and rivers it sank below. Every number in this video is sourced (below). This is real history, not a what-if. The orange on the map is our approximate shape of the land below high tide, drawn from GSI elevation tiles ("BELOW HIGH TIDE · APPROX. SHAPE"); the area we quote, 124 km2, is the city government's figure, not our measurement. The tunnel lines are an "APPROX. ROUTE". The Ring Road 7 tunnel is in the higher west and was built for rain; it was not built because the east sank. "216 pools" is today's tunnel (540,000 m3 / 2,500 m3 per 50 x 25 x 2 m pool); about 570 is the whole system when finished. "OUR MATH" and "OUR GIS" mark our own calculations and map measurements, "SCHEMATIC" drawings are not real plans, and "REPORTEDLY" marks a fact from a news source. Read it: key figures, maps and every source → https://hankomaps.com/episodes/whats-... CHAPTERS 0:00 What's under Tokyo 0:47 What Tokyo stands on 1:30 Why the east sank 3:02 The low east, mapped 3:51 Stopping a city sinking 5:48 Rain in a crowded city 7:25 Same rain, two typhoons 8:14 What else is down there 9:17 Still digging 10:08 What's really under Tokyo SOURCES • The Ring Road 7 tunnel: TMG fact sheet https://www.kensetsu.metro.tokyo.lg.j... • Why tunnels: TMG for MLIT (2024) https://www.mlit.go.jp/river/shinngik... ; Hagibis 2019, TMG (2024) https://www.kensetsu.metro.tokyo.lg.j... • Same rain, two typhoons: Cabinet Office (2008) https://www.bousai.go.jp/kohou/kouhou... • The sinking: TMG land subsidence 2021 https://www.kensetsu.metro.tokyo.lg.j... and 2023 https://www.kensetsu.metro.tokyo.lg.j... ; TMG groundwater history (2023) https://www.kankyo1.metro.tokyo.lg.jp... • The low east and its walls: TMG storm-surge policy (2025) https://www.kensetsu.metro.tokyo.lg.j... ; Koto City https://www.city.koto.lg.jp/470600/ha... ; Edogawa City https://www.city.edogawa.tokyo.jp/e00... ; 5-ward flood plan (2018) https://www.city.edogawa.tokyo.jp/doc... ; MLIT Lower Arakawa office https://www.ktr.mlit.go.jp/arage/arag... • The floodway and the Iwabuchi gate: MLIT Lower Arakawa office (2019) https://www.ktr.mlit.go.jp/ktr_conten... • Just outside Tokyo (Saitama): MLIT Edogawa office https://www.ktr.mlit.go.jp/ktr_conten... • What else: subway lengths, MLIT (2021) https://www.mlit.go.jp/policy/shingik... ; sewers, TMG (2019) https://www.opendata.metro.tokyo.lg.j... ; Roppongi (REPORTEDLY), Norimono News https://trafficnews.jp/post/131168 • Still digging: TMG link section https://www.kensetsu.metro.tokyo.lg.j... ; works briefing (2025) https://www.kensetsu.metro.tokyo.lg.j... • Pool size (OUR MATH): FINA https://artisticswimming.ca/wp-conten... Map data: Boundaries: MLIT N03 (2025), CC BY 4.0 https://creativecommons.org/licenses/... , simplified. 出典：国土数値情報（行政区域データ）（国土交通省）（https://nlftp.mlit.go.jp/ksj/gml/data.... Heights: GSI elevation tiles, processed (approx.). 地理院タイル（標高タイル（基盤地図情報数値標高モデル））を加工して作成. Relief: 出典：国土地理院（色別標高図）. 海域部は海上保安庁海洋情報部の資料を使用して作成 (sea areas: Japan Coast Guard data). Tunnel route and rivers: © OpenStreetMap contributors (ODbL) https://www.openstreetmap.org/copyright (approx.). Paris (pair episode): Ville de Paris open data (plub_carriere), ODbL https://opendatacommons.org/licenses/... . Figures: Tokyo Metropolitan Government (TMG) and others, as cited above. Maps and sections drawn by Hanko. Spotted an error? Say so in the comments. Corrections go in the pinned comment. #Tokyo #FloodTunnel #PlacesMapped