Lots of bad stuff lurks underground in the Vietnam of Platoon. Lots.
In a local village, the platoon finds several huge caches of weapons… underground.
In one hut, Chris Taylor finds a woman and her kid hiding… underground.
Just before the platoon arrives at the village, they find an elaborate bunker complex… underground.
You get the idea. There's a lot of danger underneath the jungle floor, and the guys are always justifiably wary of what may be there. While historically accurate, the portrayal of the underground in Platoon is also richly symbolic (go figure): all those surprises underground make us think of savage, underground layers, the home of dangerous beasts that feed on human flesh.
Alongside the more savage underground of NVA troops and Vietnamese villagers is the Underground where the "heads" (the guys who smoke pot) congregate. Now you're supposed to think of all those bad undergrounds when you see the Underground. The point is, despite how much the guys are having, this place is less than peachy. It doesn't help that it's dimly lit, and the guys getting stoned and drinking so they can escape reality (or become dead to it) if only temporarily. It's also eerily appropriate that King jokingly says Taylor has been "resurrected."
Besides the enemy underground bunkers and the platoon's drug den, the whole war itself is like one big underworld. Consider this: Taylor arrives in a plane that descends into the country, and the second Chris deplanes… he sees body bags. Vietnam is the realm of the dead, and Taylor may or may not get out alive.