A large-scale expansion MOD that brings the legendary Gargantuan Leviathan, a creature that once ruled Planet 4546B, back to life.
Players must harness lost Architect technology and unknown resources to confront an unimaginable threat.
The new content goes far beyond additional creatures, encompassing advanced equipment crafting with new mineral resources
and the construction of alien facilities unlike conventional player bases.
A harsher and more mysterious survival experience awaits, offering much more than new creatures alone.
New rare mineral resources, including emerald and cobalt, are introduced.
These materials provide the foundation for powerful equipment and upgrades to stand against the “legendary predator.”
2. 異星人のビルダーツールによる高度建築
2. Advanced Construction with the Alien Builder Tool
A construction device dedicated to alien technology, closely resembling Alterra's Habitat Builder.
Players can construct structures similar to the alien facilities encountered throughout the story.
3. かつての頂点生命体の遺骸 ― リヴァイアサン・フォール ―
3. Remains of Former Apex Creatures — Leviathan Falls
The remains of creatures that should never have fallen prey are scattered across existing biomes.
Something even higher in the food chain lurks in these waters, capable of hunting them.
4. 伝説の頂点捕食者「ガルガンチュア・リヴァイアサン」
4. The Legendary Apex Predator: Gargantuan Leviathan
An enormous leviathan that once stood at the top of Planet 4546B's ecosystem.
Awakened from its long slumber by the Aurora's quantum explosion, it rises from the depths of the Dead Zone.
Its overwhelming size and extraordinary biology inspire instinctive terror and awe in the player.
Return of the Ancientsおよび必須の依存MODであるArchitects' Libraryは、2024年12月の一般公開以降、両MODの合計で約23万回ものダウンロード回数を記録したことが、開発メンバーによって報告されている（2026年2月に確認）。この数字が、同一ユーザーによる重複ダウンロードを除外した回数かは不明である。
This MOD consists of Return of the Ancients (RotA) and its required dependency, Architects' Library (AL).
RotA includes creatures such as the Gargantuan Leviathan and Alpha Peeper, as well as vehicle upgrades. AL contains construction features such as the Alien Builder tool and new mineral resources, including emerald and cobalt, used to craft these structures.
The development team, Architects of the Unknown (AotU), also works on Architects' Library and the Creature Eggs Mod, which adds new creature eggs.
Development of Return of the Ancients began on December 6, 2020, with a team of four developers. The project subsequently grew through new team members and model revisions. On March 1, 2021, the entire MOD was rebuilt from scratch to adopt a new Gargantuan Leviathan model.
Simon Chylinski, who composed music and worked on sound design for the original Subnautica, also contributed to this MOD. According to AotU's official Wiki, he worked on the Gargantuan Leviathan's roaring, biting, and swimming sound effects, as well as new PDA audio. However, it remains unclear whether he is still involved in development or whether those sounds are included in the currently available demo.
In February 2018, Simon Chylinski was dismissed from Subnautica developer Unknown Worlds Entertainment following controversy over discriminatory and insulting social media remarks concerning race, gender, and other subjects.
This MOD is ultimately intended to become a fully fledged expansion with a story integrated into the base game. Most story-related locations, events, and voice lines have been excluded from the currently available demo and are planned for the final release.
However, the developers have not disclosed the details of the final story, including what its objectives will be or how it will conclude.
This MOD replaces certain story events from the base game with special sequences featuring the Gargantuan Leviathan.
Sunbeam Attack: When the Quarantine Enforcement Platform attempts to shoot down the approaching rescue ship Sunbeam, the ship's energy shield blocks the beam, preventing its immediate destruction. However, its thrusters are damaged, causing it to lose control and descend toward the ocean. A Gargantuan Leviathan then emerges from the water, seizes the Sunbeam in its enormous jaws, and drags it beneath the surface.
Neptune Rocket Attack: When the Neptune rocket is launched to escape the planet, a mysterious purple EMP-like phenomenon causes it to lose control during flight and fall toward the surface of the Dead Zone. Just before it hits the water, a Gargantuan Leviathan appears and devours the rocket, after which the game forcibly closes.
It has been suggested that the Neptune rocket attack may be intended as a form of “bad ending,” triggered when the player attempts to leave the planet without completing RotA's own storyline.
Even before its public release, gameplay videos of this MOD were posted on YouTube by major content creators, attracting considerable attention from the community. These creators were given special development builds that featured some creature models and structures absent from the currently available public demo.
Their videos played a major role in raising awareness of the MOD. However, more than three years passed between the earliest showcase videos and the point when ordinary players could access it. The prolonged restriction of development builds to a select group of content creators, along with the limited frequency of development updates, drew numerous complaints from the community.
The developers reported that Return of the Ancients and its required dependency, Architects' Library, had reached a combined total of approximately 230,000 downloads since their public release in December 2024 (confirmed in February 2026). It is unclear whether this figure excludes repeated downloads by the same users.
As of October 2026, Nexus Mods lists 2,216 Subnautica MODs. Using the combined download count of RotA and AL as a benchmark, only 26 of those 2,216 MODs have recorded more downloads.
Furthermore, most of those 26 MODs are either essential libraries required by other MODs or long-established releases that have been available for roughly six years. It is therefore notable that a standalone large-scale MOD released at the end of 2024 reached a comparable number of downloads in such a short period. This achievement illustrates the remarkable interest in the MOD, particularly its Gargantuan Leviathan, within the Subnautica modding community.
There have been repeated cases of third parties impersonating the development team and illegally redistributing the mod. At present, the only legitimate download source for this mod is the GitHub page linked through the official Discord server.
Any copies distributed elsewhere, including on Nexus Mods, are unauthorized and were not released with the developers’ approval. Do not download them under any circumstances. There have also been reports of malware being distributed while falsely presenting itself as this mod.
