The CAF issue ECW Mukluk, will certainly be the warmest pair of boots you'll ever wear in your life.
 
With an insulating interior liner rated up to -50C, with the accompanying winter weight sock, the ECW Mukluk is the perfect Artic Winter Boot.
 
Unlike other Military Issued Winter Boots, the CAF Mukluk is constructed with no Leather or hard polymers/rubbers which are prone to breaking in extreme cold. Instead, they are constructed from a soft rubber non-slip sole, a durable waterproof shell, and a low profile soft and malleable rubber heel and toe. They are relatively lightweight, and are the far better option of the USGI Mickey Mouse ECW Boot.
 
While they can be tightened over the top of the foot to secure your foot into the boot, they can also be sinched right above the heel and further upwards towards the calf, in order to trap as much heat as possible. 
 
These will last you a lifetime, and are a must-have for any Artic work or excursion.