For the world above,
when adventuring this far below the surface,
you're beyond a weekend warrior.
You're a modern-day explorer.
But if the entrance caves in,
you could just be another buried fossil.
From the jungles of Vietnam
to the backwoods of Kentucky
lie intricate underground cave systems
that contain mysterious,
undiscovered worlds.
But no journey into the unknown
comes with a guarantee you'll return safely.
In 2018,
a football team ventured 4 km (2.5 mi)
into the Tham Luang Cave in Thailand
when a monsoon caused a flash flood.
The team and their coach were stranded
in this vast cave network
for over two weeks.
This group made it out alive.
But if you're on the wrong side
of a stone wall like this,
With the known world
documented so thoroughly,
if you're looking to map new territory,
you'd better look underground.
Complex and ancient,
cave systems attract
experienced spelunkers and casual hikers
from all over the world
seeking to test their limits.
Forged by erosion and time,
it often takes up to 100,000 years
for a cave to widen enough
to let a human pass through.
But it only takes a moment
for the rock ceiling
to collapse behind you.
From 1980 to 2008,
there were 81 reported fatalities
from cavers in the United States.
Follow these steps,
and your chances of survival
might increase.
Most caves are formed from limestone,
carved out by rainwater
as it seeps through the soil,
picking up traces
of carbon dioxide along the way.
Eating away at the limestone over time,
the carbonic acid
creates a nexus of interconnected caverns,
with multiple points of entry to the surface.
So while you may not be able
to go back the way you came in,
chances are good
that you could find another way out.
As you journey further into the cave,
use bits of fabric, paper,
or create rock piles
to mark your location trail.
Look for the high ground
by searching for light or air currents,
and keep an eye out for footprints
and you maybe you'll find a way out.
Even if you're carrying
a portable water supply,
it could take weeks
to be rescued from a cave-in.
But if you find a clean source,
you can survive for over a month
on drinking water alone.
Look for drips of water
coming from the ceiling of the cave.
The rock and the soil above
purifies the water on its way down,
unlike the streams at your feet,
whose water could be filled
with contaminants.
Avoid drinking from still water sources
filled with algae,
or drip pools near bat caves
or you could end up with diseases
infecting your kidneys and liver.
And if you happen to find a spring,
drink directly from it.
The temperature of the cave
depends on its location.
A cave in Canada might average
around 21 °C (70 °F),
while a cave in Thailand
could be around 26 °C (79 °F).
If your clothes get wet though,
you could risk dying from hypothermia,
as our bodies can only handle
cold water temperatures
under 16 °C (60 °F) for up to six hours.
Bring an extra pair of clothes
so you can layer up and stay warm.
When you need to rest,
insulate yourself from the damp,
cold cave floor
and sleep above the ground
to preserve your body heat.
And if you build a fire,
make sure it's near the entrance
so the oxygen within the cave
isn't burned up.
The Mammoth Cave National Park
in Kentucky
contains almost 600 km (373 mi)
of underground passages.
If you get stuck in this maze,
it could take some time
before you get out.
Move at a moderate pace
as you search for the signs of an exit.
If there's a choice between
hiking for ten minutes around an obstacle
or climbing over it, choose the hike.
Watch out for loose bedrock underfoot
and don't attempt to climb
unstable rock walls
or you might be carried out on a stretcher,
if you're not buried first.
No matter how hard you scream,
your voice won't be strong enough
to reach the surface.
Instead,
attract rescuers by
banging rocks together
or against a stable wall.
This noise will reach the rescue teams
better than your shrill cries for help.
Stay strong and follow these steps,
and you might find
the light at the end of this tunnel.
So you walked out of the cave alive.
But the rains that caused
the collapse have intensified.
And the hills behind you
begin to crumble.
How are you going
to survive this landslide?
Brace yourself,
and learn here
on the next How to Survive.