Real life sunken cities - Peter Campbell

While touring the remains of
Ancient Alexandria, Egypt,

there are a few things that
present-day explorers should look for.

First, as you travel along
the Great Harbor,

keep your eyes open
for large columns and statues.

Across the bay to your left is the island
where the Great Lighthouse once stood.

And as you make your way through
the palaces of the Royal Quarter

and reach the area where
the Library of Alexandria once stood,

keep your eyes open for sharks.

Because if you visit this section
of Alexandria,

you'll be fifteen feet deep
in the Mediterranean Sea.

Though people are most familiar
with Plato's fictional Atlantis,

many real underwater cities
actually exist.

Places like Alexandria,

Port Royal, Jamaica,

and Pavlopetri, Greece.

Sunken cities are studied by scientists

to help us understand the lives
of our ancestors,

the dynamic nature of our planet,

and the impact of each on the other.

Water is essential for life,
food sources, and transport,

so many cities have been built
along coast lines and river banks.

However, these benefits also come
with risks,

because natural forces that can sink
the city are at their doorstep.

Take, for instance, an earthquake.

June 7, 1692 seemed like a normal morning
in Port Royal, Jamaica,

then one of the richest ports
in the world,

but when a massive earthquake struck,

two-thirds of Port Royal immediately
sank to its rooftops.

Today, many buildings
and elements of everyday life

remain surprisingly intact
on the sea floor, frozen in time.

That includes a 300-year-old pocket watch
that stopped at 11:43,

the moment Port Royal slipped
beneath the Carribean.

And during the winter of 373 BCE,

the Greek city of Helike was struck
by an earthquake so strong

that it liquefied the sandy ground
upon which the city was built.

Minutes later, a tsunami struck the city,

and Helike and its inhabitants
sunk downwards into the Mediterranean Sea.

Centuries later, Roman tourists would sale
on the lagoon that formed

and peer down at the city's remains.

Earthquakes are sudden,
unpredictable disasters

that have drowned cities in an instant.

Luckily, however, throughout history,

the majority of sunken cities were not
submerged by a single cataclysmic event,

but by a combination
of more gradual processes.

For instance, Pavlopetri,
the oldest known sunken city,

was built on the southern coastline
of Greece 5,000 years ago.

It's an example of a city
that was submerged

due to what is called
isostatic sea level change.

18,000 years ago when the Ice Age ended,

glaciers began melting and the sea level
rose globally until about 5,000 years ago.

Isostatic sea level change isn't caused
by that melt water,

but rather the Earth's crust slowly
springing back

from the released weight of the glaciers,

making some places rise,
and others sink.

The ground around Pavlopetri
is still sinking

at an average rate
of a millimeter per year.

But the ancient inhabitants were able
to move gradually inland

over several generations

before they finally abandoned the city
about 3,000 years ago.

Today, divers swim over the streets
of Pavlopetri

and peer through ancient door jams

into the foundations of houses
and community buildings.

They learn about the people who lived
there by observing what they left behind.

Natural geological events,
such as earthquakes and tsunamis,

will continue to shape our continents,

just as they have for millions of years.

As increased global warming melts
our polar ice caps at accelerated rates

and sea levels rise,

we will be forced to adapt,

like Pavlopetri's inhabitants.

Undoubtedly, over the coming centuries,

some of the coastal areas
that we live in today

will eventually be claimed
by the water, too -

Cities like Venice,

New Orleans,

Amsterdam,

Miami,

and Tokyo.

Imagine what future civilizations
will learn about us

as they swim around the ancient ruins
of the cities that we live in today.