NARRATOR：

Listen to part of a lecture in an archaeology class.

FEMALE PROFESSOR：

OK, so, as we know, archaeologists discover objects from past civilizations—

stuff like old pottery, old tools, even sometimes old bits of fabric...

and they examine these artifacts to learn about past civilizations.

But why are some artifacts preserved well enough to last for thousands of years

while others just wear away and disappear?

Well, a lot of it has to do with the environmental conditions in the area where the artifacts are found.

Artifacts are preserved better in environments where the bacteria that cause decay are less likely to grow.

So let's look at two environmental conditions that discourage bacterial growth

and thus help preserve archaeological artifacts.

One environmental condition that inhibits bacterial growth, and helps preserve artifacts,

is [slowly] aridity,

uh, lack of moisture.

Bacteria that cause decay can't survive well in dry environments,

and artifacts don't decay as fast in arid climates

without much moisture.

So, many of the best-preserved archaeological artifacts have been found in such climates.

For example, in the deserts of Egypt, archaeologists have found tombs more than two thousand years old

with brightly colored wall paintings in them.

And those wall paintings?

Well, their colors were still as clear and bright as a painting made today.

Another environmental condition is lack of oxygen.

Bacteria, like all living things, depend on oxygen to grow,

so when there's no oxygen present,

they can't grow and cause decay.

So artifacts are usually well preserved when they end up in environments that contain little or no oxygen

like, for example, the bottom of the ocean,

which is where archaeologists found an ancient ship that had sunk

and settled into the mud at the bottom of the Mediterranean Sea.

The ship was carrying vases,

and the vases were still intact and remarkably well preserved.