The temperature of the permafrost in the surrounding area of the tunnel is 1.0°C. This temperature is too warm for the frozen silt of the tunnel to support the weight of 15 meters of soil above. Therefore forced cooling has been utilized since nearly the time of initial excavation.
Initially in the wintertime, passive convective cooling was utilized. This was accomplished by drilling a vent shaft from the surface above the tunnel, to the end of the adit. The doors to the portal and a louver on the vent were opened after winter temperatures dropped continually below freezing. Natural convection currents would draft the warmer air of the tunnel up to the surface, pulling cold winter air into the Tunnel through the portal.
The vent shaft was difficult to maintain as surface and ground water would migrate around the vent shaft pipe and completely freeze the shaft shut. Over the years three shafts were drilled to maintain this cooling method. In the early 1990s the last shaft froze shut and this method has not been used since that time.