Complex sentence: This are the only truly continental Antarctica marathon races, open to all competitors, that are held at 80 Degrees South, within the Antarctic Circle, at the foot of the Ellsworth Mountains just a few hundred miles from the South Pole.
Split into simpler sentences: This are the only truly continental Antarctica marathon races, open to all competitors. They are held at 80 Degrees South, well within the Antarctic Circle, at the foot of the Ellsworth Mountains just a few hundred miles from the South Pole.