I'm building a half acre permaculture pond. It's about 10 foot deep at the center. And should hold somewhere between 300k-600k gallons of water. I dug it out with a backhoe. You wouldn't believe how scary it is driving a backhoe, it feels like you are gonna tip over at any moment.
Which region if I may ask? Did you encounter any permitting difficulties?
And no permit was pulled, building codes are fairly relaxed out in the country.