Originally Posted by
matt_hooks
I'd imagine something along the lines of the very hot water causing the air to expand, along with the heat softening any wax blocking the eustatian and allowing proper equalisation of pressures.
There is the ear drum between the eustatian tube and the outer ear and it should be intact e.g. wax can't get into the tube.
AFAIK the pain is the unequal pressures on the ear drum...