The ice was part of one of the computer-generated landscapes. The horse was filmed in the studio and the ice was added in later. Pretty impressive!
I too freaked out when the horse went through the ice, but watched to the end, and then the trailer below it on how it was made! Very good camera work and computer digitizing!
This was intense! To say the least! But you have to finish watching it and then watch the trailer on how it all came about! Super!
WOW what a fantastic film production! Intense to say the least, till you watch how it was made! Awesome!