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It's September the 14th, 2017.

In the South Pacific, just off the coast of the Cook Islands, a shadow lurks beneath the waves. The dark outline of a massive creature.

In the greenish gloom, it's hard to gauge the animal's true size.

Parts of its immense body remain shrouded in darkness,

while others catch the shafts of sunlight filtering down from the surface.

The effect is of something too big to comprehend at a single glance,

so unfathomably large it must be taken in piece by piece. The sharp pectoral fins, the dark, unblinking eyes, the broad sweep of its humped back.

The creature moves slowly but deliberately, powering itself through the water with swift, rhythmic swishes of its tail fin.

Its attention seems to be fixed on something in the murky distance.

On someone.

The question of danger is very real.

When I first started working with them underwater, I was pretty scared.

You don't realize how big they are until you're right up next to them.

63-year-old Nan Hauser kicks her diving flippers and slowly rotates her arms, treading water.

A seasoned marine biologist, Nan has spent countless hours in this ocean,

documenting the activity of some of its largest inhabitants.

...