The Fukutoshin Line is the deepest metro line in Tokyo, with an average depth of . At Shinjuku-sanchōme Station, the line passes under the Marunouchi and above the Shinjuku lines at a depth of , with a gap of only to the Shinjuku Line tunnel. The deepest section is at the immediately adjacent Higashi-Shinjuku Station, where the line goes down to , partly due to an underground space reservation for a possible future extension of the Jōetsu Shinkansen to Shinjuku.
