NARRATOR: It is widely accepted
that without access to metals,
both technology and civilization
would not have been possible.
But luckily for mankind,
some 3.8 billion years ago it
is estimated that
trillions of asteroids
crashed into the
Earth and deposited
a layer of heavy metals into
the planet's now hardened crust.
 These elements weren't
actually from Earth originally.
All of these elements came to
Earth via comets and asteroids
that impacted our planet long
ago and early in its history.
 So all the precious metals
that we mine on the Earth
actually came from
the asteroids.
NARRATOR: The bombardment of
asteroids seeded Earth's crust
with enough metals
to make possible
the Bronze Age, the
Iron Age, and today's
technological civilization.
But many metals, including
rare-earth elements
needed for high technology, are
in increasingly short supply.
Because of this, many experts
believe the asteroid belt may
once again come to the rescue.
 You get a typical asteroid
of a few hundred meters
in diameter, it will have
more of those rare-earth
elements that have been mined on
Earth in all of human history.
NARRATOR: Of the more than 6,000
asteroids in NASA's database,
it is estimated that even
just the 10 easiest to reach
and mine would yield an
astonishing $1.5 trillion
in resources.
 The asteroid belt could
provide for the needs
of our civilization
for many centuries,
maybe thousands of
years into the future.
 The natural thing to do
is to build spacecraft, go
out to the asteroids,
mine them, make
goods out of the asteroids.
And we presume that
other intelligences,
if there are other
intelligence, would
think the same way we would.
And so anything that
seems to make sense to us
could make sense to others.
NARRATOR: If other
intelligent lifeforms
exist on nearby
exoplanets, might they too
be aware of the vast resources
that exist in the asteroid belt
and also on planet Earth?
Ancient-astronaut
theorists say yes
and suggest that Earth is rich
in another commodity that would
be of great value to any
advanced civilization looking
to mine for precious
metals, liquid water.
 If aliens wanted to
mine the asteroid belt,
they'd need a base, somewhere
to regroup and refuel.
As it happens, there's
one pretty close,
and it's called planet Earth.
BILL BIRNES: Why
did aliens come here
might well be because we're
mostly water on planet Earth.
They stop here
because they could
break down water into
hydrogen and oxygen as fuel.
So if you have a craft that
somehow uses hydrogen power,
you have all the hydrogen
you'll ever need,
and it well could be
that this is a way
station for extraterrestrials.
