It's a rubber EPDM 45 MIL liner with a felt underlayment. At its deepest point it's about 3 feet. I rented a small backhoe called a Terramite to dig the hole. Spread sand over the soil, then the underlayment/liner.
I live in zone 6b northeastern PA, so the fish hibernate throughout the winter. When it gets real cold, ice will form, but the fish just hang out on the bottom.
It's a nice place to sit when the weather is warmer. We throw food out and they go crazy. I've had some of those koi for 9+ years. They started out at maybe 4" and are huge now. A few probably over 2 feet.
It's a fun hobby and once the natural bacterial cycle is established, it's really not a lot of work.