That fence in the background is fairly new, and I think it was added in 2008. This was an AZA award-winning exhibit 25 years ago, but it is now an average polar bear enclosure.
I was told years ago that right after this exhibit opened, David Hancocks (then still the director at Woodland Park Zoo nearby) visited and declared this was the best zoo exhibit in the world. Jeremy Cherafas's book "Zoo 2000" essentially said the same thing. The beautiful illusion of un-bounded space (with musk ox grazing in the background), the fantastic tundra landscape that enveloped visitors and animals alike, combined with the then-unusual underwater viewing opportunities truly made this a special and influential exhibit. In retrospect the animal area is clearly too small, and the hideous fence added in response to the San Francisco fiasco has closed the chapter on this exhibit. It's a shame.