ghostly shipwrecks,

and lost cities.

These aren't just wonders found in fictional adventures.

Beneath the ocean's surface,

there are ruins where people once roamed

and shipwrecks loaded with artifacts from another time.

This is the domain of underwater archaeology,

where researchers discover and study human artifacts that slipped into the sea.

They're not on a treasure hunt.

Underwater archaeology reveals important information

about ancient climates and coastlines,

it tells us how humans sailed the seas,

and what life was like millennia ago.

So what exactly can we find?

At shallow depths mingled in with modern-day items,

we've discovered all sorts of ancient artifacts.

This zone contains evidence of how our ancestors fished,

how they repaired their ships,

disposed of their trash,

and even their convicted pirates, who were buried below the tide line.

And it's not just our recent history.

800,000-year old footprints were found along the shore in Norfolk, Britain.

In these shallow depths,

the remains of sunken cities also loom up from the seafloor,

deposited there by earthquakes, tsunamis, and Earth's sinking plates.

Almost every sunken city can be found at these shallow depths

because the sea level has changed little in the several thousand years

that city-building civilizations have existed.

For instance, in shallow waters off the coast of Italy lies Baia,

a Roman seaside town over 2,000 years old.

There, it's possible to swim among the ruins of structures

built by Rome's great families, senators, and emperors.

And then there are shipwrecks.

As ships grow too old for use, they're usually abandoned near shore

in out-of-the-way places like estuaries, rivers, and shallow bays.

Archaeologists use these like a timeline to map a harbor's peaks and declines,

and to get clues about the historic art of shipbuilding.

At Roskilde in Denmark, for example, five purposefully sunken vessels

reveal how Vikings crafted their fearsome long ships 1,000 years ago.

When we descend a bit further,

we reach the zone where the deepest human structures lie,

like ancient harbor walls and keys.

We also see more shipwrecks sunk by storms, war, and collisions.

We're still excavating many of these wrecks today,

like Blackbeard's ship,

which is revealing secrets about life as an 18th century pirate.

But past 50 feet, there are even deeper, better preserved shipwrecks,

like the wreck at Antikythera,

which sank during the 1st century BC.

When it was discovered, it contained statues,

trade cargo,

and also the earliest known computer,

a mysterious device called the Antikythera mechanism

that kept track of astronomical changes and eclipses.

Today, it gives archaeologists vital information

about the knowledge possessed by the Ancient Greeks.

It is in this zone that we also begin to find aircraft and submarines,

such as those from the World Wars.

Plunging as deep as 200 feet,

we can find some of the earliest and rarest signs of human history.

Prior to 5,000 years ago, there was a lot more dry land

because glaciers trapped much of the water that now forms the sea.

Our ancestors spread across these lands,

and so on the sea floor, we find their camps,

stone tools,

and the bones of animals they hunted.

These sites give us invaluable knowledge about our ancestor's migration patterns,

hunting methods,

and technologies.

In the deepest zone, no human has ever walked.

This area has been submerged since well before mankind evolved.

The only artifacts we find are those that have drifted down from above,

like NASA's Saturn V rocket engines at 14,000 feet,

and the deepest shipwrecks.

The ocean is like a huge underwater museum

that constantly adds to our knowledge about humanity.

With only a fraction of it explored,

discoveries are sure to continue long into the future.