/diy/ - Do It Yourself

Hello /diy/! I have (probably prematurely) begun digging a hole on my property in hopes of making an underground room/tunnel. I live next to a old water-filled strip cut in Southern Illinois, and my current plan is to dig down and into the side of a large, 90 year-old mound of excavated dirt that has grown numerous trees that were large even when I was young. I have played +200 hours of Minecraft, so needless to say I am quite the expert when it comes to digging. Joking aside, I could certainly used some advice and links to relevant sources of info on the web on how to properly complete this project as safely as can be done for a low budget. Here are my current conditions:

1. I will be using no power tools except in the case of constructing any wooden framework needed for the structural integrity of project. All digging will be done with a shovel, a bucket, and any non-power tools that can be purchased for cheap.
2. I am performing this task on my own with no assistance when digging. Each time I go out, I have someone who will physically check on me every hour if I do not contact them in the event of an accident. This is not ideal, but it is a restriction I have.
3. My final project would ideally be a 16'x16'x8' subterranean room with necessary supports to prevent a collapse, and a lockable door to prevent unsolicited wanderers from entering and causing an accident. I'm not certain on what preventative measures to take to keep rain from entering this room or the necessary stairs/ladder shaft section to reach the door.