Stellaris: Subterranean Civilization Event Guide

Quick Links

• How To Start The Subterranean Civilization Event

• Should You Establish Communications Or Destroy The Subterranean Civilization?
  • Pre-Emptive Strike
  • Establish Communications

Despite being a 4X strategy game, Stellaris has countless story-driven narrative events that give it the depth and flavor that have made it a fan favorite for nearly a decade. A common storyline that you'll see early in the campaign is the Subterranean Civilization, where a newly-colonized planet turns out to have a sentient, industrialized species living in caverns below the surface.

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From a gameplay perspective, there's no "right" way to approach the event, but you might choose to steer it toward a particular outcome. Here, we'll break down how your decisions impact the Subterranean Civilization storyline's eventual resolution.

How To Start The Subterranean Civilization Event

Like most events in Stellaris, the Subterranean Civilization appears at random. There is a small chance of it spawning whenever you colonize a new planet, but once it appears it will not repeat for the rest of the campaign.

If a planet has a Subterranean Civilization, the storyline will begin within a few years of establishing a colony there. The exact time it takes is randomized, but if a colony goes five years without getting the Subterranean Civilization event, you can safely assume that it won't happen on that planet.

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Should You Establish Communications Or Destroy The Subterranean Civilization?

Once the Subterranean Civilization is discovered, you'll be able to carry out one of two Special Projects: Establish Communications or Pre-Emptive Strike. Establishing Communications lets you attempt to coexist with the underground dwellers, but having poor relations with them can lead to war anyway. Destroying the Subterranean Civilization via a Pre-Emptive Strike is permanent, and once you attack them there can be no reconciliation.

If your empire has the Fanatic Purifier, Determined Exterminator, or Devouring Swarm traits, destroying the Subterranean Civilization will be the only option available to you.

Pre-Emptive Strike

If you choose to destroy the Subterranean Civilization, your military will automatically eradicate them in a coordinated attack when the Special Project completes; there is no need to deploy armies or conduct orbital bombardments. Blockers will be added to the planet that, once cleared, add Agricultural, Mining, and Generator Districts as your colony repurposes the old tunnels.

Approximately 20 years after the destruction of the Subterranean Civilization, you'll get a second event; usually it will provide a small bonus like Energy Credits, but rarely you'll get space for additional Urban Districts. However, there is a small chance that instead of a reward, an earthquake will occur, inflicting Devastation on the colony.

Once the second event fires, the Subterranean Civilization event is concluded, and there will be no more events related to it.

Establish Communications

If you establish communications, you'll get a small bonus to Society Research for es long as you remain on good terms with your new neighbors, but the Subterranean Civilization will periodically ask favors of you. Additionally, their digging will sometimes cause earthquakes that harm the colony, inflicting Devastation.

The favors from the Subterranean Civilzation aren't always easy to give; they might demand that you return refugees fleeing from religious persecution, for example, or ask you to supply them with technology, inflicting a painful thirty-percent debuff to your Engineering research for a limited time.

Refusing the Subterranean Civilizations' requests or demanding they apologize when they damage the colony will harm relations with them, while doing as they ask and letting their offenses slide will improve relations. As long as you keep relations at a net zero - harming relations no more often than you improve them - they'll occasionally gift you Minerals.

If you harm relations with the Subterranean Civilization a total of three times more than you improve them, they will become hostile. You'll get an event warning you about a month before they attack. The Subterranean armies are weak compared to your advanced sci-fi troops, but numerous, and if the colony doesn't have a Stonghold or some armies brought in from elsewhere, it will likely fall.

• If you defeat the Subterranean armies, you'll be given a Special Project to destroy the Subterranean Civilization. This is identical to the Pre-Emptive Strike detailed above, but doesn't trigger a follow-up event.
• If the Subterranean Civilization takes over the planet, they become a spacefaring empire and seize control of the entire system.

If you're feeling Machievellian, you could let them take control of the system, then subjugate them in a war to get yourself a new vassal.

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