If one of your fresh colonies starts reporting seismic disturbances, and your scientists eventually figure out that something is tunneling underneath the surface, you have run into the Subterranean Civilization event. There is a whole alien species living below your colony, and the game asks you a fairly simple question: do you talk to them or do you get rid of them?
Mind that this is one of the rarer colony events. It only has a very small chance to happen 2 or 3 years after you found a colony, and you will see it at most once per game.
Talk or strike
You get two special projects to choose from. Establish Communications sends a probe down to make contact. Preemptive Strike collapses their tunnels and floods their cities with nerve gas, with casualty projections “close to 100%”. Once you commit to one of them, the other path will be closed, unless peace falls apart later (more on that below).
What talking gets you
Opening a channel gives the colony the Subterranean Civilization modifier (+10% Society Research from Jobs) and a Subterranean Contact Zone planetary feature. After that, your relationship with the aliens will develop through a series of events that can take a long time to show up:
- Famine: they ask for help. Paying 50 Food improves relations, refusing harms them.
- Refugees: a persecuted minority flees to the surface. Taking them in adds 100 pops to your colony but angers their rulers. Sending them back improves relations.
- Request for Technology: their generals want your military tech. Refusing harms relations. Agreeing costs you 30% Engineering Research from Jobs across your whole empire for 10 years, which is a lot more than it sounds.
- Earthquakes: their new cavern project shakes your colony (+10 Devastation either way). You can accept it (colony pops get -20% happiness for 10 years) or demand an apology, which harms relations.
- Gift and Xeno Anthropologists: 100 to 1000 minerals, or a chunk of society research. These only happen if you improved relations at least as often as you harmed them.
If relations go badly, they can come up to the surface and invade. Beat their armies and you get the Destroy Subterranean Civilization project, which puts you on the hostile path after all. If you lose, a brand new Fanatic Militarist, Authoritarian empire will be created that takes your starbase and every planet in that system. So do not leave that colony without defense troops if you go the peaceful route.
What wiping them out gets you
The hostile path pays off in space rather than research. The colony gets three new planetary features, each with a removable Blocker on it: Subterranean Farming Caverns, Subterranean Generator Areas and Subterranean Mining Sites. Later on the following can happen:
- Subterranean Expansion: the tunnels are cleared and the colony gets the Subterranean Cities modifier (+3 Max Districts).
- Gold and Treasury: your scouts find their gold mines and the royal treasury, 100 to 1000 energy each.
- Survivor Pocket: a sealed-off city survived the gas. Authoritarian and Xenophobe empires can drag the 200 pops to the surface as slaves, everybody else can only finish the job.
- Earthquakes: the abandoned tunnels start to collapse (+10 Devastation).
Which one is better?
For most empires the hostile path simply gives more. +3 districts is a permanent boost to that planet, the energy events are nice, and nothing on that path can cost you the whole system. The peaceful path gives a small research bonus on one colony and a chain of events where several options actually hurt you (the tech request in particular).
That being said, if you are playing a Xenophile or Egalitarian empire and gassing a whole civilization does not fit your roleplay, the peaceful path is perfectly fine. Just help them during the famine, send the refugees back if you want to keep them happy, think twice before sharing your tech, and keep some armies on that planet.
The Doorway is another colony event that can come back to bite you years later, and so is the Ancient Drones event.
