00:00Imagine trying to dig a giant tunnel through wet, sloppy mud right underneath a massive rushing river.
00:07If you make one wrong move, millions of gallons of water will flood in and crush everything.
00:12So how do engineers stop the water?
00:15They do something completely wild.
00:18They pump super cold liquid nitrogen directly into the earth and freeze the underground mud solid as a rock.
00:25This technique is called cryogenic ground freezing.
00:28Instead of fighting the underground water, engineers turn the liquid water right under their feet into solid ice armor.
00:36Here is the part most people don't realize.
00:38Underground mud isn't just dry dirt.
00:41It is actually a sponge full of trapped water that flows like a thick liquid whenever you try to dig
00:47through it.
00:47If you just dig a hole into that wet mud near a river, the whole tunnel collapses instantly.
00:54Normally, normal walls won't hold back the sheer weight of soggy earth mixed with water pressure.
00:59To fix this, workers sink a ring of long steel pipes deep into the ground all around the path where
01:05the future tunnel needs to go.
01:07These pipes form an invisible cage around the danger zone.
01:11Then they pump liquid nitrogen through those pipes at a staggering minus 320 degrees Fahrenheit.
01:16That is cold enough to snap steel like dry twigs.
01:20As the extreme cold radiates out of the pipes, it grabs the heat right out of the surrounding wet earth.
01:26The water trapped inside the soil crystals freezes solid in minutes.
01:31Bit by bit, individual frozen spots expand and crash into each other.
01:36They merge together to form one giant unbroken cylinder of solid frozen earth surrounding the tunnel path.
01:43This ice wall acts as an impenetrable dam.
01:47It blocks 100% of the groundwater and adds massive structural strength so the earth won't cave in on the
01:53workers.
01:54Now, the tunnel boring machine can drive right through the center of that frozen cylinder.
01:58It chews through the frozen mud just like a giant mechanical ice cream scoop.
02:03Right behind the cutting head, workers instantly install heavy concrete rings to line the tunnel walls
02:10before the surrounding ice ever gets a chance to melt back into mud.
02:14But here is a wild twist that engineers have to carefully manage.
02:19When water turns into ice, it expands.
02:21If you freeze the ground too fast, the swelling earth can actually crack nearby city streets.
02:27To prevent this disaster, engineers use computers to monitor temperatures down to a single degree.
02:33They balance the chill just right so the ground stays strong without lifting the pavement above.
02:39Once the concrete tunnel lining is securely locked in place, workers simply turn off the liquid nitrogen system.
02:45The earth naturally warms back up, leaving a permanent underground highway behind.
02:50Without this strange freezing trick, some of the biggest subway systems and underwater highway tunnels in the world today
02:58simply could never have been built.
03:00If you loved learning how science turns mud into solid rock, hit that.
03:04Check out our next video right over here to discover more hidden technologies
03:08that literally shape the modern world beneath our feet.
03:12Thanks for watching.
