The calmest climate in the world is in the deep Antarctic interior. It has been identified as one of the best spots on the planet to build an observatory in. We do know Antarctica is capable of colder than the -128 record at Vostok, satellites have recorded temps into the -140s.
https://en.wikipedia.org/wiki/Ridge_A
https://www.forbes.com/sites/trevornace/2018/06/28/its-almost-like-another-planet-coldest-temperature-on-earth-recorded-in-antarctica/?sh=5ca813754054