When the development team for DC Universe Online puts together a new chapter, “usually we try to do one big thing that’s neat and weird, and we’ve never done before,” says S.J. Mueller, the game’s creative director. “There’s like, 10, I think in this one.”

So, what’s the biggest? “Swimming,” Mueller says.

That’s because DC Universe Online’s 33rd chapter heads undersea with Atlantis on Nov. 8, meaning players will need some means of traversing a new open world. Daybreak Game Company’s superhero MMO has gone to Atlantis before, but only in a limited sense, warping players into dry interior settings. A new mode of travel is a big deal in DC Universe Online, and at first Mueller was wary that it could be done or would be worth the effort.

“I was like, ‘That seems like a lot, you guys,’” she recalled in a conversation with Polygon, “and they’re like, ‘No, no, no, we can do this.’”

Swimming in DCUO requires more than just a new movement animation or maybe nudging the game’s flight characteristics so it looks like players are floating and diving. New lighting had to be implemented to give players a sense of the darkness and visibility underwater. Shimmering refractions, bubble effects and intermittent sea life were needed to complete the look. And then they have to figure out how to transition everyone into a standard animation, such as using a weapon or a power, without it looking phony.

“Since we do so many different types of travel in our game already, we at least had a leg up there,” Mueller said. “But now it’s been years since we’ve looked at this kind of stuff. I think the most difficult part of it were a lot of the choices we had to make, like, well, what does the world look like when you’re in the water, what if you’re also in combat swinging your sword. It was just so many things that we had to think of.”

An hourlong livestream on Wednesday showed off the city, the initial missions, and the major characters of Atlantis:

Atlantis, not coincidentally, launches a month before the Aquaman feature film, and though its story doesn’t have anything to do with the film’s, Mueller knows that her expansion comes with more expectation and attention. That was partially behind the need to go big for this chapter, which begins with Atlantis itself, the largest new map DCUO has featured since 2013’s Sons of Trigon expansion. That entails not only building a new city, which will have plenty of verticality in addition to its large footprint, but also giving it a population varied enough to make it believable. That required all sorts of new clothing, hairstyles and skin textures for the Atlanteans.

“It’s debatably about the same size as DLC eight,” which depicted a Gotham City overrun by demons, “but rarely do we make maps this beautiful,” Mueller said. “So, I absolutely want [the game] to come back here.”

Atlantis will also introduce King Shark, a character who got a jolt of popularity following his appearance on TV’s The Flash in 2015. While Atlantis’ story breaks down to a conflict between Aquaman and Ocean Master, of course, King Shark plays a prominent role as an antihero/semi-villain straddling the hero and villain story paths. King Shark also is the leader of the Sea Changed, which are Atlanteans with both human and marine-life features — people with lobster claws, for example. And that population required plenty of attention to get just right, too, Mueller said.

An enormous sea monster rounds out the expansion as a big set piece, and the DCUO team’s fully original contribution to Aquaman lore. One of the inspirations, Mueller said, was learning that the peacock mantis shrimp has the most powerful punch in the animal kingdom. “We were like, what’s more comic-book villain than a super-punching shrimp, you know? ... And DC was like, ‘That sounds insane, but we want to see what you do with it.’ And they actually just played it very recently and they were really impressed; it was nothing they’ve ever done before.”

The story in Atlantis explores a classic conflict, which is that Arthur/Aquaman spends too much time bothering with the surface world and someone else has convinced the population that they should be in charge. This time it’s Corum Rath, the villain from the Deluge expansion this spring that sort of foreshadowed Atlantis. If players are heroes, they’ll work with Aquaman and Mera to put down the rebellion and save his claim to the throne. Villain characters will ally with Ocean Master.

Of course, that effectively means writing two storylines, with a player character as the center of attention. Mueller was on a panel at New York Comic Con recently in which she spoke about the challenge of writing a comic-book adaptation within those boundaries.

“Shannon McCormick does several voices in our game, and he said something that really resonated,” Mueller said. “It was how voice actors change their performance when they’re doing games because they’re performing with someone who’s on the other side of the screen. It really resonated with me as a game designer because that’s how we craft every experience.”

Atlantis launches Nov. 8 and is accessible to all players who are level 10 or higher. It will be part of the main game, which is free-to-play, for a limited time before transitioning into the library of premium content. DC Universe Online is available on PlayStation 4, Windows PC and Xbox One.