Those guys who are green and get you in the sunlight are called "creepers" and they're scary as hell when you're working and all of a sudden you get blown up by one that snuck up on you. Sssssssssssssssssss...BOOM! I build all my buildings with them specifically in mind.
So, i did some exploring. Dug a hole in the ground, used ladders instead of trying to create stairs to get down. This seems to work out good. So, I'm building my lair, multi-leveled, just in case the creepers do manage to survive the fall down the hole. So I'm digging away, spending "days and nights" toiling away at making a walkway to a more central area. I decide to use the ladder hole approach thing to go deeper, next thing I know I'm in this huge fucking cavern. I blocked the water flowing in, found lots of coal and iron. I was happier than a pig in shit toiling away at getting that space setup.
Alll of a sudden I hear creepers. WTF...I'm too far underground. I turn around and theres about 5 creepers running towards me. That didn't end well. They must have fallen through another hole or something and I gave up on it. I copied the save file before I dug the second ladder hole, so i might attempt again, but this time making a room to block off the creepers in that cavern.
This game is fucking epic. Seriously, this is one time when graphics don't mean shit. Stupid, mindless game with no story and it's more of an addiction than WoW has been.