ok, i know NOTHING about tanks that large except what i have read on here. so don't take anything i say to seriously but just thinking about this builds got me
anyways i've always dreamed about this type of "tank" however i'd consider this bordering on a manmade lake and speaking of once you are underground why stop at 60ft and why circular. might be easier for materials/installation to go straight edged. remember although circles are stronger they need to be made same or higher quality otherwise just as likely to fail.
i would look up making ponds/lakes since you get many advantages. i have relatives in ny(only been once) that built a pond/lake probably 75 x 50 ft over 20 years ago. its about 12 ft deep at center. they have bass in it and some have been around (i am told) for years, so much so that they have learned to follow people as they walk around it so when frogs get spooked they basically jump straight into their mouth,

sorry got distracted but point is that there is no extra filtration except the plants and sand bottom. only thing they add is to stop algae blooms. so if you built it (i'm assuming LA) you could probably set it up to be mostly self sustaining with only a few extra filters depending on how many fish are in it. my idea has been to pump the water up and then have it go through a type of river system around the edges where it will have fast and slow areas to become oxygenated then allow the bio to work. then all you have to do for viewing is basically build a plywood aquarium reversed and sink it. Simple

(j/k)
sorry to ramble, but like i said the idea is great to me. if i had land, i'd probably be digging right now

anyways best of luck and keep updating.
oh yeah and why not add some gars, you could just go catch em
