A year after the epic Planet Earth II, Sir David Attenborough is returning to our screens with a follow-up to 2001's Blue Planet.
No spoilers, but we've seen the first episode and as sequels go we can confirm it's better than Sex And The City 2.
We've also been to the press conference and spoken to some of the team behind the new series.
Here are 22 things you need to know about Blue Planet II - complete with behind-the-boat info and what you can expect from the first episode.
1. Five babies were born to crew members in the time it took to film.
Blue Planet II involved "125 shoots, 6,000 hours filming underwater and 1,000 hours filming in submersibles", explains production manager Katie Hall.
But away from the shoots, she adds, it was also a busy time for the crew: "We had three weddings. Four houses were bought. Five babies. Two babies on the way.
"That's what happens when you're filming for four years."
2. "Oceans cover 70% of the surface of our planet, and yet they are still the least explored," says Sir David Attenborough in the opening sequence.
"Hidden beneath the waves, there are creatures beyond our imagination," he continues.
Roughly translated, we think he's saying: "Look, I know you might have seen Finding Dory multiple times but this is the real deal."
3. Hans Zimmer is doing the music again.
It's bad news for Fifth Harmony fans as the soundtrack has already been sorted by composer and general legend Hans Zimmer.
"We actually put the music out to tender, we weren't sure who we wanted to do it," explains executive producer James Honeyborne.
"And although we loved Planet Earth II [which Zimmer also did the soundtrack for] we wanted to keep this feeling fresh.
"But Hans came to us with such an impassioned pitch as to why he wanted to do it, which was incredible."
4. There will be seven episodes in the series.
And they're split into different habitats - like deep ocean, the coral reefs, the open ocean and the coastlines.
"Once you start dividing those up, you start to see where some of the stories would naturally fit," says James.
5. We wouldn't fancy filling out the risk assessment form for this show.
"When we green light each shoot, we're constantly mitigating risk," says Katie.
"But bar a couple of earaches, a little bit of water at the bottom of a submersible, and one cameraman stubbed his finger, we were extraordinarily lucky.
"But the planning that goes into every single shoot, and managing the risk - whether that's financial risk or health and safety - you don't see any of that."
6. There's some serious drama in the opening episode.
Spoiler alert etc, etc.
In the opening episode, we meet some birds. Who sometimes like to fly down to the ocean and chill out a bit on the surface.
Unfortunately for them, there's some rather hungry giant trevallies (ie big scary fish) lurking under the water. And they have a particular taste for birds.
We won't give much more away, but it's safe to say you're about to see some snakes and iguanas-level drama go down.
7. Fish are Facebook-friendly.
The trailer for the new series has already built up 43 million views on social media.
Which is all part of a new digital strategy this year, according to Tom McDonald, who is (deep breath) head of commissioning at the BBC's Natural History and Specialist Factual unit.
"One of the interesting things about Planet Earth II was that more people watched it than the X Factor results show when they were up against each other last year," he says.
"So if there is a younger audience out there that is going to consume natural history content, we should focus our digital strategy on them, which is what we've done this year."
8. New technology was built to film the series.
"We built a huge piece of housing which we called the megadome which allowed us to slice the sea in half so you can see above and below at the same time," James says.
"We also built a tow-cam which can be pulled behind a boat which allowed us to travel with really fast-moving animals like dolphins and tuna."
9. They've brought back those 'making of' segments at the end of the episodes.
Remember on Planet Earth II, you'd get 50 minutes worth of TV show, and then 10 minutes of how the episode you've just seen was shot?
Half the time, the "making of" bit was even more interesting and eventful than the show itself.
Well, they've brought that bit back - and this series will have a segment called Into The Blue at the end of each instalment.
10. There's some stuff in the series that even Sir David Actual Attenborough didn't know about.
"When we first showed him some clips, he was like, 'Oh I didn't know this' and 'I didn't know that'," James says.
"Genuinely he was surprised. It's always good if you can surprise David, it doesn't happen often."
11. One Walrus has clearly been watching Titanic.
At one point in the first episode, a walrus tries to seek refuge from a preying polar bear on a floating iceberg.
Problem is, there are already some walruses (walri?) on said berg.
Clearly, this walrus watched Titanic, and, like the rest of us, thought there was plenty of room for Kate Winslet to shift over and make room for Leonardo DiCaprio on that piece of wood.
Unfortunately, the extra weight of this particular walrus on this particular iceberg causes it to collapse, ruining the party for everybody (there's always one).
12. Bottle-nosed dolphins are super-cute
They just look so friendly don't they, you'd definitely go for a drink with one.
13. Dolphins surf, Sir David explains, "to strengthen friendships, develop social skills, and for the sheer exhilaration of it".
Coincidentally, this is also why humans do karaoke.
14. The show doesn't shy away from the impact of pollution.
"You can't go out there and make a series like this without seeing some really big issues unfolding in front of you," says James.
"You can go to the middle of the ocean and find plastics from many different continents coming together.
"The impact that has on the wildlife can be devastating."
In the show, we hear Sir David comment: "There are worrying signs that conditions in the oceans that have remained relatively stable for millennia, are changing radically."
15. Warning: The first episode includes "mating".
In case you're planning to watch with your parents.
Fortunately, the mating involves this handsome chap.
16. Some shots weren't filmed on location.
Earlier this week, The Guardian said some footage had been captured in "controlled laboratory conditions".
After the report was published, James explained the series was overwhelmingly shot on location, but in a few select instances, such as rock pools in the Coasts episode, labs were used.
"Filming in the wild would have been too disruptive for the wildlife," he said.
"It would have been impossible to film close-ups of this magical world, so we worked with scientists to accurately recreate a rock pool in the controlled conditions of the lab."
17. It wasn't originally going to be called Blue Planet II.
"The series was commissioned as Oceans," explains Tom.
"It was a very easy commission, instinctively the channel understood it was a brilliant thing to do and it felt like it was the right time to go back to this habitat.
"It was never designed so that we'd have Planet Earth II last year and Blue Planet II this year, so now it looks like it was a brilliant strategy, but it wasn't exactly conceived that way."
18. Tusk fish are determined little creatures.
There is one tusk fish who travels to the edge of the coral reef every morning, Sir David explains, looking for some breakfast.
While most of us would grab some Special K from the cupboard, this guy travels quite some distance in search of a small clam.
Using his mouth, he takes it back to his "kitchen" - which is essentially a massive rock, against which he starts bashing the clam repeatedly in order to crack it open.
"It's not easy when you have no hands," Sir David observes.
19. Not all of the plans for the series came off.
"There were lots of shoots that failed, to put it bluntly," says Tom.
"The team set out to capture particular species, and when you watch this, I think James and his team make it look really easy.
"And when I think about the number of variables when it comes to filming in the ocean, I think it's remarkable what they were able to capture."
20. In true W1A style, the show has led to more collaborative BBC working.
"Some of the new technologies that were invented specifically for this series are already being deployed by the Natural History Unit on future projects," says Tom.
James adds: "There was a sequence on The One Show last week about sturgeon [not Nicola], and they borrowed our probe lens, which is great - we built it, they were able to use it."
21. There's going to be a podcast to accompany the series.
So make sure you're all caught-up on Serial.
22. So... Planet Earth II. Blue Planet II. What's next?
"We have a big landmark series for next year which is very different to Blue Planet II," says Tom.
"It's been announced already, it's called Dynasty, not to be confused with Carringtons and Colbys, so who knows if it will stay that title.
"But the premise of each film is we take one animal family and follow them for a number of years to see the ups and downs of their family lines.
"I'm already viewing episodes of it, and it's incredibly visceral. It feels so different to this."
"One other thing I'm really excited about is a series the NHU have made which is on in January called Animals With Cameras, a really simple proposition.
"We've built bespoke cameras for different species, that the animals wear, so we can see the world from an animal's point of view.
"There's meerkats in it," he adds. "That's my innovation for 2018, more meerkats."
Blue Planet II begins on BBC One at 20:00 GMT on Sunday. The series has been sold to more than 30 countries around the world.