How to get to the minecraft underworlds?

So I've been playing the game for a while now, and i've noticed a glitch in the game which is super creepy. In creative mode I start flying over land and some of the land doesn't load immediatley so i see a binch of weird rocky pathways underneath and sometimes even floating rooms and mobs. But when i fall to maybe enter this under world, the land appears underneath and the under world goes away. I tried digging to see if i could reach it (stupid idea) but obviously all I manage to reach is the Void. What is this underworld and can you get to it? If yes how? See the pic below
Thanks for your help and yes, I am a girl, i play minecraft:P

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