Swampy Biome

The Swampy Biome is a starting biome found on the Quagmiris Asteroid and serves as the core of the Swamp Start in Oxygen Not Included. It is a temperate, resource-rich biome characterized by large quantities of Mud, Polluted Mud, Polluted Dirt, Polluted Water, and patches of clean Water, plus Swamp Chard plants and bog flora. The biome contains several geysers and vents (including Cool Slush Geysers, Cool Salt Slush Geysers, a Cool Steam Vent, a Hydrogen Vent, and a Chlorine Gas Vent) as well as environmental features such as slime blobs, geodes, ruins, and occasional Minor Volcanoes or Natural Gas Geysers in nearby pockets. In game code the Marsh Biome is referred to as HotMarsh.

The Swampy Biome is messy and demands careful planning. Polluted solids in the biome behave like sandfall: Polluted Mud and Polluted Dirt can collapse and fall under the pressure of Polluted Water, and Polluted Dirt off-gasses as it degrades. The starting area contains no bacteria by default, so any beneficial or problematic microbes must be introduced by the player. The environment initially reads as temperate (roughly 10–20 °C), but the local climate can change rapidly depending on neighboring biomes; if the base is adjacent to the Tundra Biome the Swamp Start will cool quickly, so do not expect the initial temperate range to persist.

Several unique logistical and gameplay consequences follow from the Swamp Start. The abundance of slurry and polluted liquids makes breathable air easier to maintain early on because many solids release Carbon Dioxide and other gases that can be managed without immediate complex air systems. However, the abundance of polluted water means you must guard clean water supplies: digging can flood tunnels with polluted water and contaminate cisterns, so build a dedicated, sealed cistern early and consider relocating pacu or other fish into controlled tanks. Pacu may occasionally mutate into gulpfish which can help keep a cistern clean.

The Swamp Start is paired with DLC content and new mid-game buildings designed for sludge and wet resource processing, such as the Sublimation Station and Sludge Press. Its infinite low-temperature water sources—Cool Slush Geysers and Cool Salt Slush Geysers—allow refrigeration strategies that avoid early plastic-intensive systems: a Liquid Tepidizer can often be used in place of an Aquatuner–Steam Turbine combo to cool and process slush resources. Nearby Jungle Biome pockets may provide a Hydrogen Vent, Cool Steam Vent, and drecko spawns useful for exosuit production.

Practical notes and player strategies:

• Expect early-game messiness: plan containment for Polluted Water and place storage/processing infrastructure below or away from unstable muck to avoid catastrophic collapses.
• Build a sealed cistern quickly to protect clean water; house pacu or gulpfish there rather than leaving them in open polluted pools.
• Use Cool Slush and Cool Salt Slush geysers as a cold resource; they can substitute for more complex early refrigeration if routed with a Liquid Tepidizer.
• The abundance of carbonaceous materials reduces immediate need for oxygen production infrastructure, but it increases CO2 and polluted gas handling needs and causes Yucky Lungs effects on Duplicants if not managed.
• The biome contains slime and algae blobs that can be harvested, and geodes that yield Sand, Sandstone and Igneous Rock.
• Nearby resource biomes available from the Swamp Start include the Wasteland Biome, Tundra Biome, Barren Biome and Space Biome; consider expansion routes to exploit free refined Iron or other nearby geysers.
• Be aware that Sweepy and Sweepy's Dock interactions are relevant for cleanup: Sweepy cannot open closed doors or climb ladders but can pick up spills and solids up to 10 kg per tile; an Auto-Sweeper can empty solids from Sweepy's Dock but not liquids, so duplicates will be required to remove liquid spills.

The Swampy Biome gives a challenging but resource-dense beginning: with proper containment, use of cold slush sources, and exploitation of sludge processing buildings, the Swamp Start transitions from a difficult opening into a comparatively easier mid-game.