trenches in the world.

The Puerto Rico Trench at the Triangle's southern point

reaches depths of over 27,000 feet.

But Dr. Gallagher suspects that Mabel may frequent the Tongue

of the Ocean, a steep underwater trench

less than a mile away from Rose Island.

The deepest branch of the Great Bahama Canyon,

it's named for its shape that's like a tongue.

The Tongue of the Ocean drops off steeply just half a mile

or so offshore.

Goes from about 30 feet to 300 to thousands really quickly.

We really have no idea what's going on

down in the deep trenches here.

Anything could be happening in the deep sea.

It's one of the final frontiers for human exploration,

even more interesting, in my opinion, than exploring space.

JOE ROMEIRO: So animals of all sorts

can live within this trench.

And it seems like there's reports

of all kinds of different creatures living

within the tongue of the ocean.

It's sort of become this legendary spot.

NARRATOR: If Mabel is in the Bermuda trenches,

she may not be alone.

Once thought to be a barren zone with few lifeforms

able to sustain themselves without the sun's light,

marine biologists have discovered that the deep waters

of the Bermuda Triangle are home to other sharks,

from the megamouth to the sharpnose sevengill.

Among the diverse shark population in the depths

of the Bermuda Triangle, one small shark stands

out for its audacious attacks--

the cookiecutter shark, a terrifying shark

with a deceptively sweet name.

SPEAKER: Cookiecutters have very distinctive.

Their teeth are actually fused together.

Each individual tooth is fused together, unlike what

you see with other sharks.

NARRATOR: This rarely photographed shark makes

dramatic vertical migrations.

During the day, it dwells in depths over 3,000 feet.

But at night, it rises up over 2,000 feet to hunt.

SPEAKER: If you look on the underside of a cookiecutter

shark, it actually has cells that produce light,

what's called bioluminescence.

When a predator sees the bioluminescence in the water,

it goes over to it to see if anything has been disturbed.

A cookiecutter shark uses this opportunity

to latch onto the side of these predators

and take a bite from it.

NARRATOR: The cookie cutter only grows to 20 inches,

yet it has the audacity to attack whales,

tuna, even great white sharks.

The proof is in the telltale injury

the cookiecutter leaves on its victim--

a hole in the skin about this size and shape of a cookie.

If you ever looked at how they get

cookie dough out of something, it's with a scoop.

And that's basically what these sharks do.

They have this jaw structure that

can latch onto the side of an animal

and like scoop out a piece of flesh.

NARRATOR: The cookiecutter shark spends

its days deep in the ocean.

But even at shallower depths, these sharks can be

extremely difficult to locate.