Over a century ago,

the United States Fish Commission

dug a well plunging 188 feet into the earth

right at the heart of the city.

And when they did, water rushed to the surface

at thousands of gallons a minute,

flushing out dozens of undiscovered creatures

from the depths below.

These animals had never seen the surface.

Nor would they,

because all of them were blind.

Perhaps the strangest-looking animal in this catch

was a species dubbed Eurycea rathbuni,

now known as the Texas blind salamander.

Upon discovery, herpetologist Leonhard Stejneger

described them as such:

Narrator: These unusual conditions are contained

to a specific 26-mile stretch of caves

in the Edwards Aquifer,

an extensive underground layer of rocks.

It's the only place in the world

this species of salamander has ever been recorded.

So, how did these caves create the "profound differences"

in these salamanders' appearance?

In other words, how did they end up looking so weird?

To answer this question,

we have to go back in time.

Way back.

Tom Devitt: Take the dates with a grain of salt,

but we think that this group split off

from its most recent common ancestor probably around

somewhere between 10 and 15 million years ago.

Narrator: And geologists estimate that the aquifer

had been hundreds of millions of years in the making.

During the Jurassic period, some 200 million years ago,

the area was occasionally covered in shallow oceans,

but when the area was dry and exposed to air,

the rocks were eroded by weather to make porous caverns.

Eventually, a shallow basin of water and sediment

covered the land.

Then that basin dried up.

But rainfall continued to eat away at the earth

to create the deep fissures and caverns inside the aquifer.

Scientists like Devitt think

that as the climate became more arid,

some salamanders crawled beneath the surface,

seeking refuge in the cool and stable environment

of the Edwards Aquifer.

And over the course of millions of years,

these salamanders adapted to the aquatic subterranean caves

by evolving certain features,

like blood-red gills outside of their body,

while losing others, like their eyes.

Living in complete darkness

alongside prey that was also blind,

the salamanders simply didn't need to see,

and energy used for sight would only be wasted.

Today, only two small black dots just under the skin

remain as evidence of eyes past.

Those dots likely only detect if it's light or dark out.

This use-it-or-lose-it type of evolution

also applies to their silvery white skin.

Skin pigmentation usually protects animals

from the sun's UV light

and serves as camouflage from predators,

but both of those are nonissues for the blind salamanders,

who don't have to worry about the sun or predators.

These small, spindly salamanders

are at the top of the food chain in this ecosystem,

feeding on various crustaceans, like shrimp.

And even though they can go months without eating,

when a potential snack does swim by,

several features help them capture their prey.

One is a special adaptation

similar to the lateral line found in fish,

which detects vibrations in the water.

The salamanders' sensory cells respond

to changes in water movement and pressure,

letting them know what direction

and how fast their meal is moving.

Another helpful feature is their large head,

which fits 10% to 15% more teeth

than surface salamanders.

More teeth means a deadlier bite,

which is crucial if you come by prey

only a few times a year.

Danté Fenolio: All of those things are characteristics

that help you survive

in a very, very challenging environment,

so that's why they look the way that they do.

Narrator: Fenolio is one of the lucky few

who've seen these elusive salamanders up close.

Last year, his team at the San Antonio Zoo

bred Texas blind salamanders for the first time in a lab,

and they've repeated their success four more times.

They're aiming to protect the species from extinction,

since the salamanders have been endangered since 1967.

Two main things threaten the Texas salamander's survival.

The first is contamination from wastewater runoff.

This is a big issue,

because the Edwards Aquifer is a so-called karst aquifer,

which means it's made of soluble rock like limestone.

Any pollution on the surface

can rapidly seep into and contaminate groundwater.

Wastewater typically holds tons of nutrients,

which fuels the growth of algae.

When the algae decompose,

this process sucks oxygen from the water,

leaving less oxygen for our sensitive salamanders.

The second threat is a loss of habitat.

This happens when rainfall cannot adequately replenish

the water that people take,

and this threat is real.

The aquifer serves several cities,

including San Antonio,

one of the fastest-growing cities in the nation

and the largest city in the country

that relies entirely on an aquifer for its water.

So while being able to breed the salamanders

is a good first step towards one day building up colonies,

the animal may be confined to labs

if they have no home to return to.

Devitt: You know, in areas where there's been

hundreds of feet of groundwater depletion,

we've almost certainly already lost species to that,

but we just don't know about it,

because we're still describing new species

from the aquifer all the time.

Narrator: Devitt's team is using new tools

to suss out blind salamander populations,

such as a method that analyzes water samples

for traces of the animal's DNA.

He and his collaborator

recently discovered four new related species

in the aquifer.

For now, scientists say

that these salamanders can hang in there,

as long as the only home they've ever known stays constant,

but that's an increasing challenge

as more people move to the area.