How did this huge hole form?

How did this huge hole form? - Humpback whale swimming in deep sea

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.

enter image description here

minecraft-java-edition

Pictures about "How did this huge hole form?"

How did this huge hole form? - Crop woman showing pop up gift card with decorative heart

How did this huge hole form? - Pink bougainvillea near assorted sea urchins in daylight

How did this huge hole form? - Attentive woman setting up ring lamp against tinsel at home

Where Do Supermassive Black Holes Really Come From?

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: ArtHouse Studio, Michelle Leman, Julia Volk, George Milton