to big old Komodo dragons,

more than 11 million fluid specimens live

in the basement of the Field Museum in Chicago.

Josh: There are 883 frogs in here.

Narrator: But why hold on to them all?

And why keep them wet?

Think of it like a library.

Stored this way, wet specimens keep their shape

and, in some cases, even DNA --

basically, the closest researchers can get

to keeping a live zoo in their labs.

Each jar is a book researchers can crack open

and learn from, sometimes discovering

brand-new species right here on the shelves.

But you can't just drop a Komodo dragon

straight into a tank of alcohol.

The Field Museum has to acquire

and painstakingly prepare them

so they can be preserved for centuries to come.

The Field Museum acquires its specimens in two ways:

either through donations,

or sometimes researchers go out in the field

to strategically euthanize specimens

like this common water snake.

If you manage to grab one, they do not hesitate to bite.

Narrator: Sara's research relies on new

and old specimens to see how changes in habitat

have affected the species over time.

And the first thing she does with a new one

is grab its DNA.

This is a fairly new step in the process,

since DNA wasn't really used the way it is now

until the '90s.

It's not impossible,

but it is much easier to take tissue samples

from fresh animals

than it is to take them from preserved animals

and get really good results.

Narrator: She uses scissors and forceps

to collect the DNA sample,

first sanitizing them

by burning away any random DNA

so the results aren't mixed.

Sara: It does get so hot, though, that then I have to

dip it into some ethanol

so that it doesn't, essentially,

sear the animal as I work on it.

Narrator: Sara takes the sample

from the inside of the snake,

so she doesn't mess up what it looks like on the outside.

Sara: The way I'm cutting is so that

if somebody comes along,

they're still going to be able to count these scales.

Narrator: Plus, that's right around

where the liver is located.

Sara: And there's the liver! Right there.

Narrator: It's Sara's favorite tissue to collect

for DNA extraction because it dissolves easily.

Sara: It turns everything pink.

It says "SR 1291."

I double-check my tag is "SR 1291."

Narrator: Then the DNA goes into

these massive liquid-nitrogen freezers

with thousands of other DNA samples.

Now she's ready for formalin,

the liquid that preserves the tissue

and keeps a specimen frozen in time.

It's sort of like embalming it,

just like a person at a funeral home.

Narrator: Sara has to keep in mind what info she needs now

and what researchers might need in the future,

like the sex of the snake.

Sara: It's always good to see if,

maybe if it's a male snake,

if you can find the hemipenes and pop them out.

Narrator: Sara will pose the snake

so you can see the visible penises

from outside the jar without even opening it.

And this pose is how it will stay

for the rest of its afterlife.

The coil doesn't just look snaky ...

Sara: There is some art to it.

And you can stack lots and lots of them

on top of each other in a jar.

One,

two,

three, four.

I've got five snakes here,

and there's another, probably, six in this jar.

Narrator: Last step in this part of the process

is tucking it in under a formalin-soaked paper towel.

It keeps the snake saturated

without having to fill the tub.

Sara: Good night, snaky.

Narrator: Over a few days,

the formalin will set into the tissue,

leaving the snake fixed in place.

Almost like you're holding a rubber snake.

Narrator: Larger animals might need more

than a few injections,

like this catfish Caleb is working on.

Calculating the amount of formalin needed

is mostly based on experience and feel.

Too little, and your specimen will start to decay

and get floppy.

Too much, and your specimen will bloat

and become disfigured.

Caleb: You don't want to make the belly do this

because you've pumped it with so much formalin.

Narrator: Once Caleb is confident his catfish

is sufficiently full,

he'll move it into a tank of even more formalin

to soak for about a week.

Caleb: We're going to add a bit of cheesecloth

just to make sure that no parts of it

are sitting outside of the formalin.

Narrator: After the formalin, the team switches over

to alcohol baths for long-term preservation,

like with this Komodo dragon.

The alcohol is less toxic than formalin,

so it's safer for researchers in the long run,

and the specimen doesn't change much

while it sits in its final resting tank,

just the color of the liquid.

Josh: Especially large specimens,

they'll release a lot of debris and fatty oils

that were stored in their body,

and it'll leech out into the ethanol,

and that causes a lot of the discoloration.

It's still doing its job

and keeping the animal preserved.

Narrator: Most specimens in the wet collection

are kept looking as lifelike as possible.

But others ...

Caleb: We can clear away all of the tissue,

stain the bones and stain the cartilage,

and we can end up with just a skeleton

that we can put under a microscope.

Narrator: Extra-small fish have extra-small bones

that are difficult to keep track of.

So instead of isolating the skeleton,

this method keeps it contained

but visible inside the body.

First step is dyeing the specimen blue.

This specific blue dye is attracted to cartilage,

and the red dye clings to calcium.

A few days for each is typically enough to lock in the dye.

The next step is to clear the fish.

We use an enzyme called trypsin

that digests proteins and break them down,

but it leaves the collagen

that holds everything together.

Narrator: Making the fish completely see-through.

Finally, he dyes the bones red.

Caleb: One of the advantages of clearing the fish

and then putting it into the red dye

is you can keep an eye on it

to see how dark it's getting.

Narrator: The whole process can run a few days

to around a month.

Done right, and your final product

are these almost alien-looking specimens.

Caleb: It's kind of like Jell-O,

and you store it in glycerin in the end,

because glycerin and collagen

have the same refractive index,

or the way that light passes through.

Narrator: These specimens go right into collections

alongside all the opaque ones,

so researchers can access them

when all they want to see is bones and cartilage.

Caleb: You can put this under a microscope.

You can move bones around

and see how one bone moving

affects other bones nearby.

Narrator: Entirely new species

have been discovered this way,

like these two fish species

that are identical on the outside.

Caleb: But when you clear and stain them

and you look at their bones,

you can actually see that there are differences

in their skeletons between species.

Narrator: New species can hide on the shelves for decades.

Sara: So, here we go.

Narrator: Like this spider-tailed horned viper

kept under lock and key.

It was originally collected in the 1960s,

and researchers first thought it was

a different species of viper with an abnormality.

Sara: This weird, weird parasite or a tumor.

Narrator: But then ...

Sara: In the early 2000s,

some herpetologists came along,

and they said,

"You know, I think it's a whole new species altogether."

Narrator: Once a second one was found,

they compared it to this one.

Sara: And the species Pseudocerastes urarachnoides

was described from this very individual,

this holotype.

Narrator: But specimens like this one

are only helpful if you can find them.

Caleb: It's kind of like a library,

but jars of fish.

Different families of fishes have numbers,

and then within each family,

they're arranged alphabetically by genus

and then by species.

Narrator: There's even a field in the database

for noting when and where a specimen was last seen.

Caleb: So if someone asks for it, we know that,

well, at least at this date,

it was spotted on the shelf in the collection.

Narrator: And once you find it,

it's not always as simple as pulling a jar off a shelf.

Josh: There are 883 frogs in here.

Narrator: And each one has its own ID number,

so if a researcher wants to look at a specific frog ...

Josh: I have to sit here and just pick each one up

one by one and be like, "Nope, not this one.

"Nope, not this one," until you find the right one.

Narrator: These frogs used to be stored separately,

but at times ...

Josh: We've definitely run out of space.

They combined jars of the same species

but collected by different people,

collected from different places,

collected at different times,

but then you end up with the same problem I mentioned here.

Narrator: Getting rid of stuff is not an option.

Old specimens can be especially valuable.

Josh: This is the oldest specimen we have

in the amphibian and reptile collection.

Narrator: Sara has even developed a technique

for recovering DNA stuck inside them.

The process is similar to getting DNA from fresh tissue,

with a little extra work.

Sara: That extra work includes heating it up real high

and trying to digest it

and pull the DNA away from the formalin

over a much longer period of time.

Narrator: But it can be hit-or-miss.

It could be anywhere between 0% success

all the way up to, like, maybe 60% or 70% success.

If all you have is a 100-year-old snake in a jar,

you might as well give it a shot.

Narrator: Like library books,

some of these jars can sit untouched

on their shelves for years.

But all it takes is a curious person to crack one open,

and our understanding of the natural world

can completely change.

Sara: I always think whether it's DNA,

whether it's taking the whole specimen to preserve,

this thing then didn't just die in vain.

It lives on in science forever.