Here's why:

* Winter Solstice: June 21st marks the Winter Solstice in the Southern Hemisphere.

* Polar Night: During the Winter Solstice, the South Pole is tilted furthest away from the sun, resulting in continuous darkness for 24 hours.

* Duration of Darkness: The duration of darkness at the South Pole varies throughout the year. From the Winter Solstice to the Summer Solstice (December 21st), the South Pole experiences continuous darkness.