I decided I'd like a pond a few years ago and this one has been deepened a few times since we dug it. It is almost 2m in depth and the Koi seem to like it.
This one is the biggest of my Koi and is more than a decade old. I bought the fish at around 3 to 4 inches and they've grown!
My son, the Fish Whisperer.
For context, the box is 18 inches / 45 cm diagonally.