Shinekiwi's Super Awesome Things to Build - Superflat?!

Hey Minecrafters,
So I was on my world, you know, "The Everything", well, supposedly, it's superflat. While I was building my giant woolly mountains, I saw some odd shapes that definitely weren't builds. I flew closer, and there I discovered....

(This one includes a mineshaft!)

A huge chunk of a default land! This was so weird! This goes for miles, and I'm wondering if it was a glitch. It sure will be a great place to explore on survival! :) Please comment if this has happened to you before.

Have fun if you discover this!
Shinekiwi