When engineers designed the 800-mile Trans-Alaska Pipeline System in the 1970s, they faced a massive thermodynamic crisis. Hot crude oil pumped through the pipe at up to 140 degrees Fahrenheit would melt the permafrost underneath it. Thawing permafrost turns rock-hard frozen soil into soup, causing heavy infrastructure to sink, bend, and burst.
To solve this, engineers raised over 400 miles of the pipeline above ground on 78,000 vertical support posts. However, the steel posts themselves could still conduct ambient heat into the soil. The solution was the thermosyphon, a brilliant, completely passive refrigeration system embedded inside the support stilts.
Each hollow steel post is sealed and partially filled with anhydrous ammonia. During the freezing Alaskan winter, heat from the ground causes the liquid ammonia at the bottom of the post to vaporize. This gas rises to the top of the stilt, which sticks out into the cold air and is fitted with aluminum radiator fins.
The frigid air chills the ammonia gas back into a liquid, which drips back down to the bottom to repeat the process. This continuous loop acts like a thermal siphon, actively pumping heat out of the deep soil and venting it into the atmosphere. In the summer, when the air is warmer than the ground, the liquid ammonia stays safely at the bottom, automatically shutting the cycle down so heat cannot travel backward into the tundra.
Without electricity or moving parts, these clever heat pipes turn winter cold into a thermal shield that keeps the ground frozen through summer heatwaves.