Temperature (Below Zero)
The temperature in Subnautica: Below Zero will be a major environmental game mechanic that the player must manage. Extreme temperatures (both hot and cold) are harmful to the player.
Temperatures and rate of exposure can be affected by the weather conditions. The current weather and forecast can be seen in a base Control Room. Robin's current warmth will be indicated by teeth-chattering sounds.[1]
Temperature Protection
Cold
Certain consumables will boost the player's warmth, such as Heating Gel, Heat Fruit, and Coffee Americano[2]. Thermal Lilies can be found on the surface and provide warmth to the player.
The Cold Suit will exist to protect the player from low temperatures, and the PDA will warn the player about cold weather and hypothermia (extremely low heat in the body)[3]
Heat
The player starts taking damage from temperatures above 50°C. The UI will indicate that the player is too hot by showing glowing orange lines on the edges of the screen. However, the player's thermal resistance increases to 70°C if they're wearing the Reinforced Dive Suit.
Energy Generation
High temperatures can be used to generate energy for Seabases through the Thermal Plant; because these are external generators that have to be placed at specific locations to work (any source above 25°C, which can be found in the HUD of the Prawn Suit or on the Thermal Plant), the Seabase needs to be close, or otherwise connected via Power Transmitters.
Prawn Suits can generate energy from heat using the Thermal Reactor upgrade (any source above 30°C), and the Sea Truck will do the same using the Heat Recharging Upgrade[4]
Heat Sources
Black Smokers
Black smokers are a type of hydrothermal vent chimney-like structures that emit black material resembling smoke. They are found commonly in the Thermal Spires and in lower number in the Tree Spires. They produce heat that can be harnessed for energy, but are not harmful to the player.
They can also be seen in the shape of trees exclusively in the Tree Spires.
Hot Springs
Hot springs are flatbeds of water located all over the Rocket Island. Some hot springs contain Crystalline Sulfurs lining their edges. It is unknown what effect hot springs will have on the player in the future, but they will likely warm up the player.
Lava Geysers
Lava Geysers are areas of the ocean floor that spew out lava and steam. Every few seconds, the craters erupt, throwing steam and chunks of lava into the water surrounding it. They can only be observed from a safe distance, as their eruptions will cause massive damage to the player. Even approaching the area near the geyser hurts the player due to the extreme amount of heat.
Purple Vents
Purple Vents are large volcanic structures found only in the Purple Vents biome. They can be seen bursting from their rocky shell and spewing a violet stream of smoke. Unlike most thermal structures in Below Zero, the Purple Vents will burn the player when they are close to them.
Thermal Lilys
Thermal Lilys are large Flora species found only on Rocket Island and Crash site, but will most likely be found in more locations in the future. When a player is not near them they remain dormant with their petals folded around their eyes. However, when the player approaches them, they will stare at the player which will heat them up. If the player is too close to the Thermal Lily, it will slowly burn the player.
Thermal Vents
Thermal Vents are large, narrow thermal structures located in the Arctic Kelp Caves, Crystal Caves and Deep Twisty Bridges. The area around thermal vents is generally charred, showing their extreme heat output. Many vents have nearby resources such as Diamond, Lithium, Magnetite, or Gold Outcrops. It is unknown what effect thermal vents will have on the player in the future, but they will likely act similarly to geysers.