Unreal Engine 5
Subnautica 2 is built in Unreal Engine 5 (confirmed build 5.5.4 from the June 2025 playtest leak). This marks a departure from the Unity engine used for both Subnautica and Below Zero. Before Unity, Unknown Worlds used their custom Spark engine for Natural Selection 2 (2012).
Why the switch
The move from Unity to UE5 was a significant decision given the cost of rebuilding technical foundations from scratch. Unity's licensing controversies in 2023 (the per-install fee announcement and partial walkback) may have influenced timing, though the transition likely began earlier. UE5 offers several technologies directly relevant to an underwater exploration game.
Technologies in use
Behavior trees and stimulus system
UE5's behavior tree system powers creature AI. The Collector Leviathan's custom "stimulus" system responds to light, sound, and player actions in real time. AI Lead Antonio Munoz Gallego designed it so creatures "constantly re-evaluate the situation" rather than running scripted loops.
Control Rig
Unreal's Control Rig system handles complex creature animation. The Collector Leviathan's tentacles use fully simulated physics through Control Rig rather than canned animations, creating unpredictable interactions during player encounters.
Dynamic weather system
Dev Vlog #1 showed a dynamic weather system affecting ocean currents, visibility, and creature behavior. Previous Subnautica games had static weather. In an underwater game, weather changes can alter swimming difficulty, creature spawns, and visibility in biomes, adding unpredictability to exploration.
Resource node system
Dev Vlog #1 also revealed that the engine supports a new resource-specific node system with a decimal-based inventory (e.g., "0.71 titanium"). This granular approach to resources is a technical departure from the whole-unit crafting system of previous games.
VR compatibility
The playtest leak showed compatibility with UEVR (Universal Unreal Engine VR mod), achieving 90 FPS on high settings. However, the developers have stated VR support is "not currently in development." Any VR experience would come from community-built solutions.
Performance concerns
The community has raised concerns about UE5 performance based on issues in other UE5 titles, particularly shader compilation stutter and traversal stutter when loading new areas. These are known engine-level challenges that developers address through precompilation and streaming optimization.