Its entirely possible.
I know of two similar places here in the UK!
One is a disused quarry, the work ceased in the quarry when they hit too many springs to bother pumping out the water, so they left it to fill.
This is probably several 100 feet deep in some places....
http/www.miac.org.uk/gullet.htm and i can say from first hand up close and bloody scary experience there are one helluva lot of extremely large koi type fish in there.... and probably a lot of other stuff but once you have come eyeball to eyeball with a big carp you stop swimming underwaterw ith your eyes open and adopt a 'if i cant see it, it cant slurp my toes off' policy.
The other place is near where i live now (sorry no pretty weblink), its the outfall pools for the water used for cooling in a dye works. The water is directed from a stream, through the factory and is pumped out into a series of smallish sheltered ponds, the top pond staying at a fairly constant 22c even during winter and a little higher in summer.
This is FULL of tropical fish, some koi that appear to like the temperature (id guess because it is a larger pond and quite deep there are cooler areas and obviously they wont suffer the same problems of poor water quality as its far more diluted than a garden pond or fishtank would be), but there are also terrapins in there and definatley some cichlids...
So if its possible here, id say its possible elsewhere!