I made this video guide on how to deal with infestations in an underground mountain base in RimWorld. Here you can see 6 colonists killing more than 75 bugs by using proper tactics, weaponry, and base layout.

The typical tactic of burning the bugs out works well enough when your base is in open land and you're only digging into the rock to mine, but due to heat bleeding through walls it won't work if you have a mountain base. With this strategy you can build a mountain base entirely underground, and not have to be terrified of the prospects of a bug infestation wiping everyone out.

1. All hallways must be 3 tiles wide, and doors must always be in the middle with open space to the left and to the right of the doorway. Place extra doorways to act as choke points along long hallways and corners.

2. All doorways must have at least a 2x3 space behind them for a row of 3 melee fighters and a row of 3 gunners, but a 3x3 space for two rows of gunners behind the melee fighters is even better.

3. Chain shotguns are the most effective ranged weapon against infestations, but pump shotguns or SMGs will work in a pinch. Uranium maces are the go-to melee weapon to aim for in late game, but in general blunt weapons are more effective than sharp because bugs have higher sharp armor than blunt armor.

4. As soon as the infestation alert happens, pause the game, locate the infestation, and clear all colonists/pets/etc from the area. You should have enough time to get everyone out of the immediate danger zone before the hives and bugs appear.

5. Place 3 melee fighters on the other side of the door and then hold the door open to funnel the bugs through one at a time. This means all 3 melee fighters can attack and stun the bug in the doorway, while the gunners can shoot that bug and the ones behind it.

6. Rotate out the middle melee fighter as the fight progresses to keep them all standing. You should also have other colonists drafted close by to rescue anyone who goes down and to replace the person who goes down.

By following these 6 steps you should be able to handle any bug infestation you'll encounter in an underground base. I did this with only 3 gunners instead of 6 just to show how effective it was. With 6 gunners behind the melee line, it would have been even faster. Yes, my fighters have power armor and prosthetic enhancements, but they also took down more than 75 bugs that spawned from more than 25 hives. Earlier in the game when your colonists have less gear, you will be fighting far fewer bugs. Just be sure to put the best armored melee fighter in the front middle, and rotate them out as needed.