On her very first dive into the frigid waters of Antarctica, Orla Doherty’s yellow submarine began taking on water. So she did what she was supposed to — she stuck a finger in the puddle at her feet and licked it. It was salty. That meant it wasn’t drinking water spilled by one of the crew members; it was saltwater leaking into the sub — at 1,476 feet (450 meters) below the surface. “That made my heart start beating quite fast,” says Doherty, a producer for BBC America.

Doherty braved the perilous Antarctic waters for the TV series Planet Earth: Blue Planet II, which premieres in the US on January 20th. The sequel to the 2001 The Blue Planet takes viewers into a seven-episode tour of the world’s oceans, from coral reefs to the bottom of the sea. Ed Yong at The Atlantic called it “the greatest nature series of all time,” and it’s hard to argue with that statement. With its mesmerizing shots of bioluminescent creatures and deep-sea dwellers straight out of a sci-fi comic book, Blue Planet II will change the way you see the ocean. If you thought fish were boring, wait until you see a tuskfish use tools to open a clam or a female kobudai morph into a male.

Over four years, Doherty and the BBC America crew spent over 6,000 hours diving underwater alongside scientists from all over the world. The Blue Planet II footage is leading to 13 scientific papers, Doherty says, about everything from Mobula rays hunting schools of lanternfish, to silky sharks and blacktip sharks rubbing against whale sharks to clean themselves.

“All that really says to me is that we did the job right,” Doherty says. “It’s very, very heartwarming.”

Doherty herself spent 500 hours underwater, becoming one of the first humans to ever venture 3,280 feet (about 1,000 meters) beneath the surface off the tip of the Antarctic Peninsula. Her first expedition with the leaking sub started on the wrong foot, but ended well: within 20 minutes, she and the rest of the crew were able to seal the faulty pressure gauge, and continue their descent into what she calls “the black void of the deep ocean.”

The Verge spoke with Doherty about the series, the new camera technology used by the crew, and how humans are changing the oceans.

This interview has been edited for brevity and clarity.

There are so many nature documentaries out there. What makes Blue Planet II stand out?

The ocean covers 70 percent of the surface of our planet. But in fact, when you look at it in terms of its volume, it’s actually more than 90 percent of the living space on planet Earth. We are an ocean planet. It’s just that our lives are on land, therefore our attention goes there. But really, if you look at the Earth and think about it in terms of sheer statistics, we’d actually be spending an awful lot more time looking at the ocean and learning more about what lives in it and what kind of struggles they’re facing. The ocean isn’t just one giant blue thing. It’s full of incredible and different worlds. I think we’ve taken the time in this particular series to focus purely on the ocean. That’s what to me really makes it stand out.

You spent 500 hours underwater to film the series. What was the most breathtaking moment?

The standout for me is always going to be the day we went to visit the methane volcanoes. We happened upon a place, because a scientist led us there, where we witnessed these giant eruptions of bubbles, bigger than a basketball, shooting out of the deep seafloor. And this is all in the Gulf of Mexico, just off the coast of Louisiana. We witnessed something that no human had ever witnessed before: these really violent eruptions. It felt to me like that day I left planet Earth, got into a spaceship, and landed on some completely different planet. And at the end of the day, I somehow made it back to Earth again when we touched down on the ship. But of course, I hadn’t left our planet at all. All I’d done is take a very spectacular dive into the deep ocean. It just made me even hungrier to find more of those scenes, more of those incredible worlds, that are going on right beneath us all the time. It’s just, we can’t be there all the time so we don’t really know what’s going on.

What is the coolest discovery in your opinion?

There were so many things that scientists thought were happening but that we’ve actually filmed, therefore we get to see them with our own eyes for the very first time. A very good example of that is the Humboldt squid. It’s been known for a long time that Humboldt squid eat each other, because scientists have analyzed their stomach content when they get to the surface. But we got to spend time down in the deep, in their world, and we were lucky enough to capture this whole cannibalism tug of war scene going on right in front of us. We didn’t even get one squid eating another squid. It was one squid eating another squid and then a third squid comes in and attacks the first squid to steal the second squid. It was big squid wars at the bottom of the ocean. And this is going on all the time: something like 30 percent of their diet is other squid, but it’s never been seen. To me, that was unbelievably exciting.

What kinds of new technologies did you use in the filming of Blue Planet II?

Some of it was state-of-the-art camera sensors that could really respond to filming in extremely low light conditions. That’s what I used in Chile to film the Humboldt squid, because we wanted to try to make ourselves invisible. The other thing that we had to do: there was this really cute fish in the Deep episode, a sea toad. He’s pink and he lives right down on the seafloor, and I really wanted to get down and look him in the eyes. But all the cameras on the submarine are up on platforms, so everything we were shooting from the submarine was looking down at him and that’s not what I wanted to do. I didn’t feel like we were getting to know this guy if we weren’t really looking at the world on the level that he’s seeing it at, and looking at him at that level. So we had to design a whole camera system that could be deployed off the submarine and then loaded down onto the seafloor to get the shot of him. That took a lot of trial and error, trying to get that right. And that was just me in the deep. We’ve taken technology to places where it was probably never meant to go.

The last episode of the series examines the impact we’re having on the oceans. Why did you decide to include a whole episode about that?

We never set out to make a series that was going to be an environmental campaign or anything like that. But the reality is, we wanted to make a series that shows the ocean and all the incredible characters that live in it. But as our filming continued, we kept coming across all these different ramifications of what we’re doing and how that’s impacting landscapes in the ocean, how it’s really changing the lives of the animals that live in the ocean. Even things like noise. The best way to do that in the final episode was to really show all these scientists — these amazing, amazing people — who are out there trying to understand what’s happening, what’s changing, why is it changing? Ultimately we all need a healthy ocean. The ocean is what makes planet Earth a habitable planet, for us, for all the creatures that live in it. I think we’re just beginning to wake up to that fact.

What do you hope Blue Planet II will accomplish?

I hope that it will show people how incredible and how unbelievably beautiful the oceans are and how many different kinds of worlds there are within them. When you take a journey across the whole series, as you go from week to week, I just hope that the audience will be kind of amazed by what’s going on out there because it is the majority of our planet. My hope is that the most lasting impression that people will take from it is, “Wow, what an incredible place our ocean is.”

Planet Earth: Blue Planet II is premiering in the US tomorrow, January 20th, on BBC America. It will be available for free for a month on BBCAmerica.com and the BBCA app after it airs.