Complex sentence: The'Glacier returned to retrieve the devices, but the world's mightiest ice-breaker was unable to break through the ice, although it progressed far enough to extend helicopter fly-overs.
Split into simpler sentences: The'Glacier returned to retrieve the devices, but found the ice too thick. The world's mightiest ice-breaker was unable to break through the ice, although it progressed far enough to extend helicopter fly-overs.