Interview
July 15, 2026
Subnautica 2: Unknown Worlds on Building Cross-Platform Co-op with Epic Online Services
Few survival games have captured players’ imagination quite like Subnautica. Developed by Unknown Worlds, the original game invited players to explore the depths of an alien ocean world—blending exploration, crafting, and survival into an experience defined by discovery and atmosphere. Since its release, the series has built a passionate global community and sold millions of copies across multiple platforms.
Now the studio is preparing the next chapter having switched to Unreal Engine 5 for Subnautica 2, a highly anticipated sequel that expands the universe with a new planet, new creatures, and—for the first time in the series—online co-op gameplay. With the team having successfully launched in Early Access to over 600,000 CCUs across platforms, we spoke with the developers about how they approached bringing multiplayer to a series built on solitude—and the technical decisions behind making it work.
Now the studio is preparing the next chapter having switched to Unreal Engine 5 for Subnautica 2, a highly anticipated sequel that expands the universe with a new planet, new creatures, and—for the first time in the series—online co-op gameplay. With the team having successfully launched in Early Access to over 600,000 CCUs across platforms, we spoke with the developers about how they approached bringing multiplayer to a series built on solitude—and the technical decisions behind making it work.
Building on a Beloved Solo Experience
Courtesy of Unknown Worlds
As you began working on Subnautica 2, what expectations did you feel—from both the community and internally—about evolving the experience?
Jack Smith - Technical Director: Our biggest challenge was building on the success of the first game without changing the core of what made Subnautica so unique and successful. With Early Access, we wanted to express several new concepts and ideas at launch and then partner with our community and players to shape how those new ideas and features evolve. (Chief among those new ideas was making Subnautica 2 something that) could be shared with their friends.
Subnautica has traditionally been a solitary experience. What drove the decision to pursue co-op? Did community demand—including mods—influence your decision to support multiplayer?
Smith: Some of the most successful community mods for Subnautica and Below Zero introduced co-op. It was often the most-requested feature amongst our fans. The challenge for us was integrating co-op in a way that did not diminish the single-player experience. Internally, we often describe the game as an emotional, immersive, exploration-driven experience you can share with your friends—as opposed to a multiplayer version of Subnautica.
Players have always talked about their most memorable moments in the game—the first time they encountered a leviathan, the feeling of building the Cyclops for the first time—and we wanted them to be able to share those experiences directly with friends in Subnautica 2.
Courtesy of Unknown Worlds
What were the biggest design challenges in adding co-op while preserving Subnautica’s tone and tension?
Smith: Our biggest challenge was diffusing the old adage of “safety in numbers.” Tension needs to exist even if you have someone swimming by your side.
The ocean setting really helps with this. It's full of unknowns—even in our own present-day reality. Harnessing that and ensuring it comes across to players is one of the ways we preserve that tone and tension.
We would argue that staring down into the abyss beyond the edge of your flashlight beam as you slowly burn through your oxygen is equally terrifying alone as it is with your friends.
Courtesy of Unknown Worlds
Choosing a Proven Foundation Built for Scale
What drove you to choose Unreal Engine for Subnautica 2?
Smith: (We) knew there would be challenging technical requirements around large open world gameplay, higher graphical fidelity, and co-operative multiplayer.
Unreal Engine provides solutions to all those requirements, and the features we were going to rely on were battle-tested in some of the biggest games on the planet. This helped the team feel confident in moving to a new engine.
Once you committed to Unreal Engine, how did Epic Online Services factor into your multiplayer planning?
Smith: As we fleshed out our requirements for multiplayer in Subnautica 2, we knew that crossplay was something we really wanted to bring to our players, even during Early Access. Our planning process involved looking at potential solutions that helped us achieve this.
Epic Online Services (EOS) stood out due to how battle-tested it is—being able to see the features we needed being used at scale in other titles helped us plan with greater confidence.
Epic Online Services stood out due to how battle-tested it is—being able to see the features we needed being used at scale in other titles helped us plan with greater confidence.
Jack Smith, Technical Director, Unknown Worlds
How did going with EOS help the team stay focused on gameplay rather than backend infrastructure?
Smith: Our multiplayer requirements are relatively light, but for a lot of the team, this was their first experience building a multiplayer experience across multiple platforms. EOS was well
documented and integrated well into the existing Unreal Engine architecture, which meant our
engineers were able to spend time ensuring that our flow and the player experience it enabled were robust.
Building Cross-Platform Session Architecture with EOS
How did EOS Lobbies help you stand up invite-based grouping quickly?
Smith: By using EOS Lobbies, we were able to have a single, consistent “backing” session that worked exactly the same across all our platforms. It meant we didn’t have to worry about how we connected people across multiple platforms at the transport/synchronization level and could instead focus on what we wanted that invite-based grouping experience to be from a player perspective.
Courtesy of Unknown Worlds
Why did you choose an invite-only model rather than public matchmaking? Was this a technical decision, a design decision—or both?
Smith: This was a design decision. At its core, we wanted our multiplayer to be a co-operative experience that you shared with your friends, and we didn’t think public matchmaking would provide that core experience.
This design decision did allow us to follow a simpler technical architecture than would’ve been possible if we supported public matchmaking, but it was not the driving factor.
Courtesy of Unknown Worlds
Can you walk us through how a player creates and shares a LobbyID?
Smith: To support cross-platform play, we need to maintain two sessions—our EOS “default” session and the “platform” session (such as Steam or Xbox Live),because we need players to still be discoverable and joinable in the platform-level Friends/Presence UIs.
This means that we need to “mirror” our sessions to ensure that the important aspects of both default and platform sessions are synchronized. (As part of this), we built on top of the existing Common Session Subsystem in UE5 and extended it to support our cross-platform session “mirroring” logic.
What can you tell us specifically about how they helped?
Smith: The requirement to mirror sessions wasn’t one that was immediately obvious to us as we worked through our initial technical designs. In hindsight, it probably should have been but we’re all learning!
We worked closely with the EOS team to define the problem and what a solution could look like over the course of a couple of calls directly with the team at Epic. This really helped us understand what our implementation needed to look like to be successful.
How did the Online Subsystem factor into your implementation?
Smith: The Online Subsystem is at the core of our implementation. It provides us with a common API that abstracts away all the different nuances of each platform we need to support, and lets us focus on the high-level flow. Asynchronous online service interactions are always fraught with edge-cases and potential for painful, difficult-to-reproduce bugs, so having a common architecture is very important.
You mentioned the EOS team has been helpful in working through the implementation. Could you explain how?
Smith: The requirement to mirror sessions wasn’t one that was immediately obvious to us as we worked through our initial technical designs. We worked closely with the EOS team to define the issue and what a solution could look like over the course of a couple of calls directly with the team at Epic. This really helped us understand what our implementation needed to look like to be successful.
Courtesy of Unknown Worlds
How did you approach ensuring players could invite and join seamlessly across ecosystems?
Smith: Players can join a game with their friends through two mechanisms: platform social features (i.e. friends overlays/menus) and friend codes.
Friend codes are a custom implementation that (lets players) maintain a cross-platform list of friends by sharing their unique friend code.
Joining any Subnautica 2 multiplayer game requires one piece of information: the EOS
SessionID. This connection is provided by the EOS P2P interface, which is the fundamental layer that supports our cross-platform connectivity.
Courtesy of Unknown Worlds
Players Showed Up in Droves — and They Brought Friends
Subnautica 2 launched into Early Access on May 14. How did the launch go?
Smith: It surpassed all of our expectations. We sold 2 million copies in the first 12 hours, and crossed 4 million sold globally within five days of release. And that saw our peak concurrent players exceed 600k across Steam, Xbox, and the Epic Games Store. Seeing the response to Subnautica 2 from all the players who have already dived in has been incredible, and we’re so grateful to see the feedback continue to flow in!
How did players respond to co-op specifically?
Smith: Players responded amazingly! They’ve currently given us a “Very Positive” review rating on Steam, and we hit 93% positive across more then 65,0000 reviews. What’s been great to see is that more players than expected have been playing co-op with their friends, creating memorable moments together as they explore the depths. We worked very hard to make Subnautica 2 a single-player game people could play in co-op with their friends, without diminishing the single-player experience.
What would you do differently if starting again?
Smith: Apart from building the session-mirroring architecture as part of our initial solution, there’s not much we would do differently. We’re really proud of what we’ve been able to achieve and how it empowers the experience we want our players to have!