Japan’s Seikan Tunnel: The World’s Longest Undersea Tunnel Japan’s Seikan Tunnel is the longest undersea tunnel in the world, stretching over 53 kilometers, with 23 kilometers running beneath the ocean floor. Built to withstand earthquakes and harsh seabed conditions, it connects Honshu and Hokkaido. Completed in 1988, the tunnel stands as a triumph of civil engineering, safety planning, and geological mastery—especially in one of the world’s most seismically active regions. Important Details: •