Those underground bases you're asking about are actually bunkers, and they're pretty crucial for running a successful weapons operation. There are 11 different spots you can set up shop in, and choosing the right one makes a big difference (location really affects your profit margins because of delivery times).

The whole operation is more complex than it might seem at first, because these bunkers actually serve multiple purposes. You'll be manufacturing weapons, managing supply runs, and even conducting research to unlock new gear. The cool thing about this setup is that it functions like a proper business operation, where everything you do contributes to your bottom line (try to always buy supplies when you're starting out, it saves valuable time).

Good to know: what makes these particularly interesting is how they tie into the bigger picture of running an illegal empire. The research you do here unlocks special modifications and weapons that can help with other business ventures, so it's actually quite strategic in the long run. Just keep in mind that some bunker locations are definitely better than others, especially when you're making those longer delivery runs to earn the big money.