Subterrania

Dig beneath your planets. A Freight Elevator sinks a shaft and opens a cave below, a whole new map under the one you know. Tunnel through the rock, mine what lies there, and build where the land above has run out. The deeper you go, the richer and stranger it gets, and the more dangerous: the caves belong to the Underground Horrors.

Works with or without Space Age.

The levels

Each level is its own surface, unlocked by its own research.

• Nauvis, levels 1 to 3 (Excavation, Deep excavation, Abyssal excavation). Solid rock to dig through. The ore patches above run on below, even ones you've already mined out, leaner the deeper you go. Pools of spring water and geothermal vents lie hidden in the rock. Granite veins get thicker with depth, and level 3 has big granite patches and pockets of pegmatite.
• Nauvis, level 4, the magma layer (Space Age, Core excavation). Open ground: a floor of volcanic ash among huge granite columns and lava pools, with coal, calcite, granite lying in the open, rocks, fumaroles, and stalagmites. At Vulcanus's pressure, foundries, big mining drills, and turbo belts can be made here. Tungsten still comes from Vulcanus.
• Under Gleba (Space Age). Rich crude oil fields and patches of glowing Cave fungus.
• Under Fulgora (Space Age). Room for your recycling. A dark plain among bedrock columns, with small patches of iron, copper, coal, and stone, a little crude oil, granite, and the half-destroyed bases of settlers who got here first. Reclaim their buildings.
• Under Aquilo, two levels (Space Age). The first is frozen like the surface, so buildings need heating: permafrost underfoot, patches of methane clathrate (a fuel that crushes into ice), veins of deep ice, and the brine, fluorine, and crude oil from above. The second is warm from geothermal heat, so nothing freezes, and at a pressure of 600 cryogenic plants, fusion, and quantum processors all work there. It has pools of liquid water, more geothermal vents, and cryolite in the granite. The Electrolytic Cell, built only there, turns lithium brine straight into lithium plates and cryolite into fluorine.

Pressure rises with depth (1000, 1400, 2000, and 4000 on level 4), so recipes with pressure conditions follow it. There's no sun underground.

On Fulgora and Aquilo, shafts go only through solid ground, not through the ocean or anything floating on it.

Digging

• Tunnel Boring Machine: a vehicle that tunnels by itself while you ride along.
• Laser Boring Tower: cuts tunnels, clears out round itself, or cuts a star of eight tunnels, with circuit control.
• Cave Bulldozer: drive it straight through the rock.
• Crawler Dock: its Vein Crawler hunts down granite veins and brings the granite home.
• Seismic Thumper: from the first level down, flattens cliffs and collapses nests on the surface above, or keeps new nests from settling. Underground, it shakes loose stray rubble.
• Quarry: a garage whose trucks drive out and mine the ore in front of it.

Making things

• Granite, crushed into quartz and feldspar. With Space Age, pegmatite gives spodumene for lithium, long before Aquilo.
• Deep Forge: steel straight from ore with no fuel, industrial diamonds, fused quartz, and silicon wafers.
• Pressure Assembler and the Mineralogy science pack.
• Hot Press Furnace: an electric furnace in a steel furnace's 2x2, with one module slot, for where space is tight. Made underground, built anywhere, and an upgrade planner target for stone and steel furnaces.
• Technical ceramic: feldspar and quartz fired in the Deep Forge.
• Cheaper alternate recipes for circuits, solar panels, low density structures, concrete, and rails. The usual recipes still work.
• Geothermal Tap and Wellhouse: steam from the vents. Riser: pumps a fluid between levels.
• Infinite research: Mineral productivity and Feldspar turret damage.

Foul air

Underground, pollution has nowhere to go. A chunk's polluting machines stop when its air gets too foul (a map setting), and start again once it clears. Exhaust Tunnels and elevator shafts let it out to the level above, and an Air Gauge shows how bad it is.

Underground Horrors

The enemy below, in place of biters: grey on the first level, orange on the second, red on the third, and bigger the farther you dig from a level's first elevator. They sleep in caverns until you break in, and they go for anything of yours they can see. Their bite crushes stone walls. On level 4 they're blue, five times as strong, and walk over lava. Under Aquilo, Frost Horrors spit a freezing stream. On the open levels, Brood Mothers lob their young at you, and the horrors spread as biters do.

Fight back with shattering quartz ammunition, feldspar walls and gun turrets, lures that draw the horrors away, resonance drones, and, with Space Age, the Long Range Missile Turret.

Cruise Missile Turret: the artillery of the caves. Artillery's long lob can't clear a cave roof, but a cruise missile flies itself: it climbs out of its silo, turns, and flies level to a nest or worm up to 120 tiles away. It fires by itself or at the artillery remote's target, and charts the ground where its missiles land. Its missiles are long range missiles with a radar, a ceramic radome, and an artillery shell's charge, in plain, explosive, and atomic. It has its own range and damage research, and works with or without Space Age.

Choose where you dig under Fulgora. There, the nests get tougher with distance from the map's starting point, not from your elevator: grey near the start, orange from about 300 tiles out, red from about 800, and the red ones are three times as strong and their nests dense. Horror size follows the level's own evolution, which starts at zero and rises with time, pollution, and nests destroyed. So a first dig far from the starting island meets many small red horrors and tough nests at once. Your first elevator does clear the nests within 150 tiles of its foot. Recyclers pollute down there too, though Fulgora's surface has no pollution, and that draws attacks.

Settings

• Underground Horrors (startup): off for no enemies underground at all.
• Horrors climb open shafts (startup, off by default).
• Peaceful horrors (map): horrors only fight back.
• Underground pollution limit (map).

Good to know

• Rocket silos and artillery turrets can't be built underground (the Cruise Missile Turret can), and mech armor can't fly over undug rock.
• Map markers and chat pings are copied between a planet and its levels.
• Other mods can add their own planets through the subterrania-planets mod-data.