00:00Imagine trying to dig a massive highway tunnel directly underneath a raging deep river.
00:06One tiny crack in the wall and millions of gallons of water instantly crush the entire crew.
00:12Yet engineers do this all the time.
00:15But here is the startling hidden truth.
00:18Before the giant metal drill even touches the dirt, they freeze the actual mud and groundwater solid as steel.
00:26This secret technique is called cryogenic ground freezing.
00:30Instead of pumping out wet sand and water, engineers turn the earth itself into a massive load-bearing ice dam.
00:38How do you freeze tons of underground mud?
00:41You certainly cannot put it in a kitchen freezer.
00:44Instead, workers sink a ring of hollow steel pipes deep into the wet ground around the tunnel path.
00:51Through these metal pipes, engineers pump liquid nitrogen at a mind-boggling minus 320 degrees Fahrenheit.
01:00That is cold enough to shatter steel like glass.
01:03As the super-chilled nitrogen races through the pipes, it acts like a thermal vampire, sucking every drop of heat
01:10out of the surrounding dirt, sand, and groundwater.
01:13Within just a few weeks, the natural water trapped between grains of sand freezes solid.
01:20The loose mud transforms into a massive, interlocking wall of solid, rock-hard ice.
01:26Now comes the real magic.
01:28With the wet earth completely frozen solid, the soil acts like concrete.
01:33It can hold back the heavy weight of the river above without collapsing.
01:37The giant tunnel boring machine can now munch right through the frozen dirt.
01:42The spinning steel teeth carve through the ice, just like a blender crushing ice cubes for a drink.
01:47Right behind the cutting head, workers immediately install heavy concrete rings to line the tunnel walls, locking the structure in
01:55place before the surrounding ice ever has a chance to melt.
01:58If engineers did not freeze this water first, the wet sand would instantly run like liquid sugar, swallowing the expensive
02:06machine and trapping workers in a muddy tomb.
02:08This clever freezing trick is so powerful, it is used under historic city buildings, subway stations, and ancient riverbeds all
02:17over the world, without cracking a single street above.
02:20Once the permanent concrete tunnel is fully built and sealed tight, engineers simply turn off the nitrogen flow.
02:27The underground ice slowly melts back into normal groundwater, leaving the tunnel intact.
02:32But here is a fascinating twist.
02:34Sometimes engineers do not just use liquid nitrogen for short tunnel jobs.
02:39In some mega projects, they keep the ice walls frozen for years using giant industrial refrigerators.
02:46Think about that for a second.
02:48Deep beneath bustling city streets, hidden miles away from daylight, massive blocks of artificial permafrost hold up entire rivers while
02:57people commute overhead.
02:58Without this mastery of extreme cold, many of the world's most famous underwater transit systems and deep metro lines would
03:06literally be impossible to build.
03:09Human ingenuity never ceases to amaze when we use physics to bend nature to our will.
03:15If you enjoyed discovering this frozen underground secret, hit that.
03:19Check out our next video right over here to learn about other mind-blowing construction secrets.
03:24Thanks for watching and stay curious.
