(narrator) On April 15, 1912, an unsinkable ship named the Titanic hit an iceberg and came to rest nearly four kilometers beneath the surface.

Light--none.

The temperature-- 2 degrees Celsius.

Pressures--5,000 pounds per square inch.

But more than 100 years later, this watery graveyard is somehow teeming with life.

Those strange icicle shapes covering the Titanic are full of microscopic organisms that thrive in one of Earth's most inhospitable environments.

They can literally eat metal.

And someday soon, they'll leave nothing but a rusty pile of powder where the ship once was.

These deep-sea microbes are extremophiles, one of countless organisms living hidden in Earth's most extreme habitats, adapted to conditions where, until recently, we figured life couldn't exist.

Extremophiles have changed how we view life's possibilities on Earth.

They hold clues to how life may have taken hold on this planet and also give us hints about life's possibilities deep in space.

Off the Galapagos Islands, two kilometers underwater, Earth's mantle and the ocean directly meet, creating strange smoking vents with temperatures above 100 degrees Celsius.

Yet, they're home to ecosystems as rich as any rainforest.

At the base of this deep-sea food chain is a weird kind of single-celled life-- archaea.

When it was discovered by Carl Woese, it completely redrew the tree of life.

They look a lot like bacteria--prokaryotes-- but archaea have unique internal machinery.

And in Earth's most extreme habitats, we find them more often than any other life form.

Organisms that are specially adapted to high temperatures can grow above 120 degrees Celsius.

It's hot enough to disintegrate most cell's machinery.

The microbes at these deep-sea vents have unique adaptations like specially wound DNA and putting extra bonds in their proteins to keep everything from melting.

And it's not just single-celled life.

Larger organisms like tube worms and hairy crabs thrive in these super hot ecosystems too.

This is a place completely devoid of light, where energy must instead be harvested from hydrogen and sulfur gases bubbling from the tectonic vents, not unlike conditions we expect to find on Jupiter's moon, Europa, where the geologically active interior creates pitch-black oceans of liquid water beneath its icy surface.

When it comes to pressure, we don't know what life's limits might be.

The deepest places probed on Earth, like the Mariana Trench, are home to microbial life able to withstand pressures more than 1,000 times higher than what we feel on Earth's surface.

And when scientists expose other microbes to low atmospheric pressures like those on, say, Mars, many were like, "No problem.

This is fine."

But there are a couple of things it seems that life can't do without.

The universal needs for life are good old carbon and water.

Life is basically organized chemistry.

Inside every cell on Earth, the making and breaking of bonds, building cellular machinery, copying DNA, even the membranes that keep a cell from spilling its guts, all depend on liquid H2O.

But salty environments, frozen environments, or low-pressure atmospheres lack usable H2O.

They're essentially as dry as deserts.

Yet, in places like super dry Antarctica and deep in hidden caves, we find microbes tucked away inside rocks and crystals where they've carved out tiny water-filled pockets, little microscopic oases in deserts made of stone and salt.

In places like Chile's Atacama Desert, one of the driest places on Earth, microbes pluck water molecules right from the air and make their own liquid shells.

On a planet like Venus, where it's just too darn hot for water to remain liquid at the surface, microbial life could be suspended in tiny droplets of water in the upper atmosphere.

One of the biggest risks to life anywhere is dangerous radiation-- UV, gamma rays, and x-rays-- which can damage cells and mutate DNA.

We don't worry about it much here because our magnetic field protects us.

But elsewhere, life would either be forced to shield itself underground or figure out how to put up with a daily dose of mutation.

Microbes seem to have this figured out too.

In places like Chernobyl, we found bacteria that can withstand huge doses of radiation.

Even cockroaches can handle at least 100 times more ionizing radiation than humans can, although this is surprising to no one.

If these extremes seem harsh, it's probably because animals like us have a very narrow window of survival.

Life has existed on Earth for more than 3 billion years, and it's flip-flopped from super scorching to super snowball many times.

Our extremes may have been normal to Earth's earliest inhabitants.

Even our oxygen-rich atmosphere would be considered extreme to some life forms.

There's a good chance the first life forms were similar to what Woese discovered at those boiling black smokers beneath the Galapagos.

Understanding how life survives our extremes broadens our horizons for where we think life can exist and tells us where to look beyond Earth.

So far, we only found life in one place.

But if the odds of sharing this galaxy with another living planet ever seem too extreme, just remember that life, uh, finds a way.

Stay curious.