Abstract: A method of, and apparatus for, constructing a submerged tunnel in situ and a submerged tunnel formed thereby has an inverted arch formation to resist upward buoyancy thrust and a sealed chamber unit mounted on the tunnel and longitudinally displaceable along it for servicing the exterior of the tunnel. The tunnel is constructed in situ by progressively lengthening towards each other two tunnel sections 3, 3' which extend in cantilevered manner from opposed base foundations 1 and 2. Each tunnel section has mounted on the free end thereof of an outer chamber unit 13, 13' and an inner chamber unit 6, 6' within the latter of which the free end of the tunnel section is progressively formed and extended. As the tunnel sections are lengthened their associated chamber units are displaced longitudinally and sequentially to provide clearance for further lengthening, such displacement may be effected by hydraulic rams or mechanical rack and pinion devices.