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00:00Planet Earth weighs about 6 sextillion tons.
00:04It has a surface area of 197 million square miles
00:09and a diameter of between 7,900 and 7,926 miles.
00:15If it were possible to drill through it,
00:18you'd pass through the crust, the mantle,
00:21the outer and inner cores,
00:23and then the same again in reverse.
00:26Would you make that journey?
00:31First off, what is Earth actually made of?
00:34At its simplest, it consists of crust,
00:36either oceanic or continental,
00:38with sections known as tectonic plates
00:40covering the entire surface.
00:42The crust is then always made of rock,
00:44but of different kinds of rocks in different places
00:46and at different layers.
00:48Below this topmost layer, there's the mantle,
00:50which is Earth's magma source.
00:52And then, finally, we have the core,
00:54both the inner and outer.
00:56The core is a thousand miles down,
00:58incredibly dense, and billions of years old.
01:01Here, at the heart of our planet,
01:03it's made of super-hot, super-dense iron and nickel,
01:06and temperatures can be more than 10,000 degrees Fahrenheit.
01:10We know, then, that inside the Earth
01:12isn't exactly a welcoming place,
01:14but nevertheless, humans are still intrigued
01:17by all that's below us.
01:19Ideas about the Earth perhaps being hollow
01:21and hosting unknown ecosystems
01:23have appeared in science fiction for a long time,
01:26but although hollow Earth theory doesn't actually hold up,
01:28there certainly are strange
01:30and genuine underground realms
01:32that are intriguing, even bewitching.
01:34The deepest known natural cave on Earth
01:36is Virovkina Cave in Eastern Europe.
01:38At about 10 miles long
01:40and more than 7,000 feet deep,
01:42it's an extreme and unique environment.
01:44If it were an ocean instead of a cave,
01:46its depth would put it firmly
01:48in what's known as the bathypelagic zone,
01:50with pressure hundreds of times greater
01:52than what's found at the surface.
01:54Unfortunately, and perhaps unsurprisingly,
01:56some explorers have died
01:58inside Virovkina Cave.
02:00Ultimately, though, it's very likely
02:02that Virovkina is even deeper than we currently know,
02:04and also that there are other,
02:06far deeper cave systems out there
02:08that haven't even been discovered yet.
02:10What really puts the scale of the Earth
02:12into perspective, however, is that in all cases
02:14we are still a long way away
02:16from reaching down into even Earth's
02:18second layer, the mantle,
02:20let alone further still to the core
02:22and out the other side.
02:24Do man-made caves offer any improvement?
02:26To some degree, yes.
02:28Virovkina Cave is about 1.3 miles deep,
02:30but the deepest mine in the world,
02:32the Mpana Gold Mine in South Africa,
02:34is almost double that,
02:36reaching 2.5 miles.
02:38If we abandon traversable underground systems,
02:40i.e. if we look at the deepest
02:42locations but ones that humans
02:44can't travel down into,
02:46then we come to what's truly the deepest
02:48man-made hole in the world.
02:50The Kola Superdeep Borehole in Russia.
02:52It is more than 7.6 miles deep,
02:54and is a real benchmark
02:56in the field, given that it was
02:58a drilling project conceived solely
03:00to see how deep it was possible
03:02to drill into the Earth's crust.
03:04Thankfully, the borehole is only about nine inches
03:06wide and is now sealed off,
03:08so there's no danger of anybody falling into it.
03:10But even if you did, you'd still,
03:12and again, be way off how deep
03:14you'd need to go to reach the core
03:16or to emerge out the other side.
03:18There's simply nothing on Earth
03:20currently that takes us even close.
03:22The middle of the core
03:24is about 4,000 miles below the surface,
03:26meaning we'd need to dig
03:28an 8,000 mile tunnel
03:30to punch through to the other side of our planet.
03:32The longest still-in-use tunnel in general,
03:34going across the Earth and not down
03:36into it, is the Delaware Aqueduct
03:38in the U.S., but it is only about
03:4085 miles long.
03:42We'd need, then, almost 100 times more,
03:44and it would all have to
03:46withstand untold pressure and temperature.
03:48In reality, drilling
03:50through the Earth, as simple as it sounds,
03:52isn't something that's yet technologically
03:54possible for humanity.
03:56But say that money is no object,
03:58some kind of tech solution is discovered,
04:00and that the entirety of our species
04:02has, for some reason, decided that we
04:04really, really need to create this passage.
04:06What then? First,
04:08we'd need a material not only to build
04:10the tunnel out of, but also our
04:12drilling apparatus. Again, that
04:14material would need to be capable of withstanding
04:16extreme temperatures and depth.
04:18There are those highs of around 10,000
04:20degrees Fahrenheit, while the pressure
04:22would top out at about 3.6 million
04:24atmospheres. For context,
04:26the deepest known point in the ocean,
04:28Challenger Deep in the Marianas Trench,
04:30has about 1,000 atmospheres
04:32of pressure. Even it just isn't
04:34comparable. When thinking of material that
04:36could work, you might immediately go to
04:38diamond, which can cope with up to
04:40five times as much for the pressure.
04:42However, the melting point of diamond
04:44is a paltry 7,200 degrees
04:46Fahrenheit, so it would inevitably
04:48melt partway through our journey.
04:50Which is a shame, because a diamond
04:52drill for a diamond tunnel seems
04:54like it would be a pretty cool thing.
04:56Scientists have researched
04:58this specific problem, however, and before
05:00come up with a new material that combines
05:02hafnium, nitrogen, and carbon
05:04– hafnium being a lesser
05:06known transition metal. But even
05:08this fares only slightly better
05:10than diamond, with a melting point
05:12of 7,460 degrees.
05:14Even our most thoroughly
05:16researched custom materials, then,
05:18only get us to the edge of the
05:20Earth's core. And, of course, only in
05:22theory. Staging any kind
05:24of practical test for what we'd need
05:26– some kind of new wonder metal or
05:28element – is basically impossible.
05:30But, hypothetically, say we do have
05:32such a material, what other obstacles
05:34would our Earth-splitting tunnel face?
05:36The consistency, or rather
05:38inconsistency, of Earth is another
05:40problem. Drilling through rock is one
05:42thing, and in fact is the only thing we
05:44know… but eventually you get to the
05:46mantle, full of heavy, molten lava.
05:48So, in carving our tunnel, we would need
05:50a craft that can drill through the ground,
05:52yes, but it would then need to become
05:54the most extreme submarine ever,
05:56capable of wading through magma.
05:58And then it would need all of those
06:00temperature and pressure resistance
06:02qualities in order to survive the core.
06:04Anything that could do all of that
06:06would be some machine. But,
06:08hypothetically, say we do have
06:10such a machine. Next, we'd have
06:12severe navigation problems, since there
06:14would be literally zero visibility
06:16for the very vast majority of our
06:18mission. And then, the heat
06:20remains a major issue, even with an
06:22indestructible, hypothetical wonder material.
06:24Because, just because something won't
06:26melt when subjected to thousands of
06:28degrees, doesn't mean it won't get
06:30so hot inside that humans can't
06:32survive. The conditions on the inside
06:34of our tunnel through Earth would be
06:36hellish. More than that, they would
06:38boil you alive. So, here's hoping
06:40we develop some ultra-advanced climate
06:42control systems, as well. But maybe we
06:44have all of that, too. We've got our
06:46tunnel made from an impossible material,
06:48the incredible machine needed to actually
06:50build it, some super-advanced air
06:52conditioning, and some sort of vehicle
06:54that can withstand the entire journey.
06:56How long would the journey take?
06:58Again, we have 8,000 miles
07:00to traverse. The fastest train in
07:02the world is currently the Shanghai-Maglev
07:04in China, which travels at 286
07:06miles per hour. At that constant
07:08speed, we're talking nearly
07:1028 hours to tunnel through the planet,
07:12which is longer than it would take to just fly
07:14between the two points on an airplane.
07:16But here's where one final and crucial
07:18consideration comes into play. Because when
07:20drilling through the Earth, we're actually not beholden
07:22by how fast trains, or anything,
07:24can move on the surface. The force of gravity
07:26messes all that up. As it's
07:28pulling everything directly towards the core
07:30all the time, we know that the journey to
07:32the centre of the Earth would, in reality, be
07:34much shorter. It would potentially take
07:36less than an hour. However, there
07:38is a catch. And a big one. Because
07:40with gravity pulling you in, how can you ever
07:42hope to get away? Your journey
07:44from the centre of the Earth, the second half
07:46in terms of plane distance, just isn't
07:48going to happen. Unless you manage
07:50to conduct the entire journey also in a
07:52total vacuum, then no matter what you do,
07:54gravity will always mean that
07:56it's actually impossible for you to leave.
07:58So, even if you've managed to build a
08:00tunnel to withstand the pressure, which means
08:02you haven't been crushed, you'd still find
08:04yourself hopelessly cast adrift,
08:06trapped in a tomb of your own making,
08:08waiting to starve and eventually
08:10die. Which isn't really
08:12the ending we were hoping for. But what do you
08:14think? Would digging a tunnel through the Earth
08:16be worthwhile? Or would it just be completely
08:18absurd, even if it were possible?
08:20For now, the want to explore
08:22is usually to be applauded. But sometimes,
08:24even the most fundamental laws
08:26of physics are telling you that it's probably
08:28a bad idea. And that's
08:30what would happen if you drilled through
08:32the Earth. What do you think?
08:34Is there anything we missed? Let us know
08:36in the comments, check out these other clips
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