Lake Vostok? The Russians contaminated it while trying to get a pristine sample, brilliant freakin’ work.
That “cathedral” idea is highly intriguing btw.
I visited the Ice Caves on Mt Rainier many years ago before they last closed up. It was spectacular. Blue cathedrals. It was really breathtaking.
Under a few miles of Antarctic ice there would likely be no light getting through... But who knows? Old, hard ice does tend to get clearer as it gets older... Maybe there are conditions that let some, if even very
Iittle, illumination get through? Dunno.
There’s a similar unnatural feature at the South Pole. It was formed from the station effluent. Frozen and strangely beautiful.