A bluntnose sixgill shark arrives to feed on the carcass of a sperm whale in the Atlantic Ocean (Photo: Will Ridgeon)

Each episode of Planet Earth: Blue Planet II, airing Saturdays on BBC America, is special in its own way, of course. But there’s a reason this weekend’s installment, “The Deep,” is truly exceptional — and it’s not just because we see sixgill sharks devouring a whale carcass or learn what happens when you have a leak in a sub as you’re 450 meters below.

The fact is, we know more about the surface of Mars than we know about the difficult-to-reach deep ocean — the majority of the living space on Earth. As episode producer Orla Doherty tells Yahoo Entertainment, “A deep ocean scientist that I spoke to very early on in the production said to me, ‘Orla, if aliens came down to Earth, they’d land on land, but they would look in deep oceans and say, ‘Those are the most abundant, most prevalent organisms on the planet. These must be the oldest occupiers of this planet,’ and us little human specs would be kind an insignificant blip, frankly. That gave me this sense of perspective of, ‘Wow, this is an enormous world, and it’s one that we just know so little about.’ We don’t know how it functions. There are amazing scientists trying to figure it out. But it’s got to be fundamentally connected to everything else in the ocean, and therefore to us, and we’re only just scratching the surface of what that really means.”

Here, Doherty offers a preview of the most memorable sequences in “The Deep,” premiering Jan. 27.

Yahoo Entertainment: You and your team were the first humans to ever dive to 1,000 meters in the Antarctic. As we see in the “making of” segment at the end of the episode, you were a half hour into your first descent when the three of you in the sub noticed a small puddle forming. How did you remain calm? Orla Doherty: I’ve had a lot of time at sea. I’ve had a lot of situations happen. I’ve been in storms, I’ve been in cyclones, I’ve been in all sorts of places, and I’ve learned that panicking is not gonna get you anywhere. It’s not my natural instinct, anyway, but it’s just not gonna do anything. So okay, I’m in a tiny submarine and there’s water coming in. There’s not a lot I can do. But what I can do is do my best to just work with the pilot who does know what to do and assist him. I’m very used to responding to commands in an emergency situation.

Another standout sequence is when the team films sixgill sharks, who may only feed once a year, tearing into the carcass of a dead sperm whale on the Atlantic Ocean floor. Why was that a behavior you set out to capture? We did it with a bunch of scientists because studying these whale falls is one really clear way to show how our life here up at the surface is connected to what happens down in the deep. Scientists have studied these whale falls in the Pacific Ocean before — in the original Blue Planet there were some great shots of a whale fall in the Pacific. But nobody had ever done it in the Atlantic. … We were really curious to know what was gonna happen, what animals were gonna come in. And it was absolutely astounding to see these beautiful sixgill sharks. On my very first three sub dives, I met a sixgill shark each time, and so I felt really connected to that animal. I think it’s a magnificent, beautiful, ancient animal. And to see them come in and be a little bit less graceful, a little bit less gracious, and just rip the carcass to shreds, and sort of wrestle each other out of the way, it was just phenomenal. And you know, the scientists learned that it was within 25 minutes that the first shark came in, so these animals are constantly on the move and have extreme sensory capabilities to be able to find food when it’s there. So it was an amazing piece of discovery — not just for us, not just for the footage, but also in terms of really understanding how that ecosystem works.

BBC Earth released a video (above) showing the sixgills pushing the sub because they, at first, considered it competition for the food. Were you in the sub when that was happening? No, my assistant producer was down there in that scene. It would have been amazing. I dived on that whale carcass a little bit further down the line. I was off somewhere else doing something else at the time that was all going on.

The episode talks about how even four months later, there are still zombie worms feeding on what’s left of the whale. It must have been really satisfying to be able to keep going down over a long period of time. That was the whole purpose, to be able to do this repeat study and to be able to see what happened over the passage of time. I think it was something like 12 or 14 months between the first and the last dive that we really got to see how this carcass just slowly falls apart and becomes recycled back into the system.

Producer Orla Doherty in the submersible Nadir, capable of reaching depths of 1,000 meters (Photo: Luis Lamar)

Another jaw-dropping sequence: the cannibal Humboldt squid, 800 meters down off the coast of South America. As a viewer, it’s shocking to see them turn on each other when they’ve run out on lanternfish to prey on. What was it like for you to witness? It was shocking to be in the sub and see that all play out in front of us! I mean, it was just incredible. We had to use lowlight cameras, like really sensitive camera sensors, to be able to film those scenes because we were part of the first [team] to ever really try and document the natural behavior of these squid in the deep ocean, in their world. Any time they’ve been filmed before it’s been when they’ve come up to the shallows and have been filmed by scuba divers.

So we were on a bit of a stakeout down there. This was all off the coast of Chile. And first we saw them hunt the lanternfish, and that was amazing — just incredible to watch them hunt with such precision and move in such coordinated ways. And then, one day we went down and there weren’t any lanternfish for them to hunt — that’s when we saw a sort of squid tug-of-war over another squid. And it was just extraordinary because scientists have known for forever that Humboldt squids eat Humboldt squids, because they’ve analyzed their stomach contents when they’ve been caught in fisheries, but nobody’s ever actually seen that happen. It’s something like 30% of their diet is made up of other Humboldt squid, so they’re mean, nasty cannibals, but to actually have it demonstrated right before our very eyes and be able to capture it as a scene, it was just extraordinary. We all have imaginings and fantasies about what goes on down there, but we’ve just got a very privileged glimpse of something that is going on over and over and over again.

When you’re watching something like that from the sub, is there a lot of shouting, like, “Whoa!” Or is it all just stunned silence? [Laughs] I’m reasonably vocal. I’ve been known to scream with excitement or to “woop” or whatever. Usually whoever’s working the camera is a lot less verbally responsive — he would then just focus on really getting the shot — and the pilots are just always focused on where we are and are they lined up right. They’ve got jobs to do. I’ve got a big job to do down there, which is look for things to film and figure out the story as we’re filming it, and figure out which shots we need and all of that. But I don’t have anything physical to do when we’re filming, so I’m much more able to engage with the animal and just be reactive. It’s just fun to be down there and seeing things that you know people have never seen before. It’s extraordinary.

When this episode aired in the U.K., the clip of the cutthroat eel going into toxic shock from a brine pool in the Gulf of Mexico circulated online. Did you know what you were witnessing when it was writhing around? I knew the brine pool was this lethal lake at the bottom of the sea, which is a head spin in and of itself. I had a whole storyboard of a really complicated bunch of shots that was gonna try and illustrate just how toxic it was. I can’t actually even remember what it was I was thinking of doing down there. But again, it was on our first dive there, and that eel took its dip in the lake, and within 45 seconds had just demonstrated perfectly that this is how toxic the lake is.

I talked to just about every scientist probably that’s ever dived and done research there, and nobody told me about the eels and the reactions that they would have. So no, we got the surprise of our life. That was yet another moment where I — actually, I think all of us were cheering and just all going, “Come on! Come on! Come on! Get out, you can do this! Shake it off!” Rooting for that eel to get away, which he did.

Not all of the creatures do, though. There’s a sort of graveyard of the fish and things that didn’t make it out. That must have been even eerier in person. Yeah, and some of them were still in the sort of death throes, and it was kind of awful. The whole thing is just such a head spin: You feel like you’re in air because you’re looking at a lake, but you’re not in air, you’re underwater, 750-meters deep. And yet I felt like if I could just get out there and scoop this thing up and lift it up into what looked like the air — but it’s not, it’s water — it will survive. Really, really painful to watch. But that’s the ocean — that’s just part of the circle of life down there.

The team also captures amazing shots of volcanic eruptions in the South Pacific. Is that all real sound, or is that enhanced? I was desperate to use genuine sound recordings. Those are genuine sound recordings from the eruption, which is just incredible. Recording sound in the deep is really difficult because if you’re in a submarine you’re making a lot of noise, so you’re therefore going to get mostly the sub’s noise.

The other thing is I worked with scientists that would try to deploy hydrophones to the bottom of the Mariana Trench, the deepest point on Earth, to find out what the sound is down there. And I was like, “Wow, great, fantastic. You’ve gotta give me those recordings when they come back.” And when the scientists got those recordings back they said, “Orla, you just don’t want them.” And I said, “Why, why?” They said, “Because all you can hear is the sound of ships. The sound of ships 7 miles above at the surface passing.” We’re making noise in ways that you could never believe, and it’s because sound travels so much further and faster underwater.

We’re just beginning to learn how much animals in the ocean, from the coral reefs really, use sound. Sound is part of their world, and we’re changing that world because of the sound we’re making.

There’s the sequence in episode 7, when clownfish (which make alarm sounds for each other, and to try to scare off predators), can’t hear because a boat is overhead. Exactly! Exactly! We didn’t know this stuff even maybe 5, 10 years ago. There’s so much going on down there that we’ve just got no idea about.

Speaking of sound, the music is so beautiful in the entire series, but it’s so interesting in this episode creating suspense in the darkness. We talked a lot with Hans [Zimmer] and [the team at Bleeding Fingers] about how this has gotta be a sci-fi voyage through these magical, wonderful worlds where you meet all these alien creatures. The moment that sticks out for me, and it’s one of the more subtle pieces of music in the film, is when the sub actually does finally touch down on the sea floor. And the music that they wrote for that moment… I just almost well up every time I watch it because the music completely enhances the sense that we have now landed on another planet —  but we haven’t, we’re here on Earth. It’s incredible.

I love the idea that the episode ends on, that if there’s abundant life existing down at those extreme depths in our oceans, and we know that there are deep seas on the moons of Jupiter and Saturn, then it’s possible there could be life there, too. That actually cycles all the way back to your very first question — why should we care about what’s in the deep ocean. Because how astounding is that? If the sun goes out tomorrow, we’re all gonna die, everything on land is gonna die, everything in the shallow ocean is gonna die — but life is gonna go on down at the hydrothermal vents. That’s just extraordinary. You take away our energy source that we are all utterly dependent on, and yet life will go on. So yes, to think that there is stuff going on down there that could inform us about how to find life elsewhere in this amazing universe is just mind-bending!

One final question: People who watch the end credits will notice James Cameron is given a special thanks. Why is that? He’s been someone that I’ve known and talked to about the ocean for nearly 20 years. He’s an ocean soulmate that has explored in an even deeper way than I probably ever will, and he has just been a guide for me. A guide and a friend.

Planet Earth: Blue Planet II airs Saturdays at 9 p.m. on BBC America.

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