So, as you can see here, I have this hollow and wide hole! I know a Creeper didn't blow it up, because of this.
- The bottom is really smooth,
- I'm in Creative Mode
- My game is set to peaceful.
This is an example of a basin. A basin is essentially an area of land not covered by grass, therefore exposing things such as dirt, gravel and ores. Your basin looks like it's pretty small, but I've seen ones that small before. Here's an example of a large generated basin.
(source: cursecdn.com)
It seems to be normal. I looked up caves on the Minecraft Wiki, and it showed a photo around the middle on the page, showing "An above ground circular cave" that looked like the one in your photo. It has some resemblance to your photo, but it was a bit bigger. Nothing weird. Naturally generated.
It is a Circular Room that is created "above" ground