Inside the World's Longest Underwater Tunnel: How the $27B Impossible Tunnel Was Built!

Welcome to MegaBuild Files. 240 meters below sea level, beneath the Tsugaru Strait, Japan spent 24 years drilling the longest undersea tunnel ever built — 53.85 kilometers through volcanic rock that flooded at 80 tons of seawater per minute. 34 workers died. The cost overran the original budget by 12 times. The tunnel opened in 1988. Then the airlines arrived. Today, 90% of passengers fly over it. The fastest bullet train in Japan slows to half speed inside it because the freight trains sharing the bore can't survive the air pressure. The Sapporo extension that was supposed to justify the whole project has been delayed to 2038. This documentary goes inside the full engineering story of the Seikan Tunnel: the flood that buried five years of work in two months, the chemical sensing system invented underground while the water rose, and why Japan keeps running a tunnel that almost nobody uses. Subscribe to MegaBuild Files to follow every megaproject, engineering crisis, and construction milestone that's quietly reshaping civilization. #SeikanTunnel #UnderwaterTunnel #JapanEngineering #TunnelConstruction #Shinkansen #Megaproject #Engineering #Documentary #Construction #UnderseaTunnel ⚠️ IMPORTANT ⚠️ This video is an educational analysis of the Seikan Tunnel's engineering and construction, based entirely on publicly available open-source information. All visuals and animations are 100% synthetic, AI-generated conceptual representations. No real archival footage was used. This video is purely informational and intended for educational purposes only.