Abstract: A re-entrable duct is provided through which a cable is adapted to penetrate a wall, e.g., of an environmentally controlled manhole. The re-entrable duct includes a duct pipe, e.g., a length of steel pipe, through which the cable passes. A resilient annular plug having a central aperture through which the cable passes, e.g., a so-called split rubber duct plug, is situated at each end of the duct pipe. The annular plugs with the duct pipe define a sealant chamber. A quantity of bentonite is provided which substantially fills the sealant chamber. A sleeve is secured to one end of the duct pipe adjacent one such split rubber duct plug, that sleeve including an end plate having a central aperture therethrough which the cable passes. The cable is thus secured within the central aperture in a circumferential, hermetic sealed manner, thereby defining a secondary sealed air chamber between the split rubber duct plug and the end plate.