There is: a 12-foot-wide moat. A trip to the roof, offering vantage points a visitor doesn't normally get, causes a grizzly to glance up curiously -- and reveals a hidden infrastructure that allows for a heavy snowfall to be plowed out of the moat, lest the bears take advantage of a suddenly shorter wall to go AWOL.
At other points, the exhibit has been engineered so that huge lumbering grizzlies with what Fisher calls "Freddy Krueger claws" will commune nose to nose with young kids, with nothing but glass between them.
That closeness is in fact "a huge engineering feat -- engineering that's hard to find, to be honest with you -- with up to seven layers of glass" all melting into one another so as to appear to be almost not there, said Karen Marshall, of the Philadelphia-based zoo design firm CLR Design, which is helping create a new polar bear exhibit at the Como Zoo in St. Paul.
Kids visiting the Apple Valley zoo have been known to wonder what would happen if all three bears suddenly hurled themselves at the glass -- and that's a question zoo officials ask themselves as well, said Milwaukee's Beehler.
"To simulate 500 pounds of lion crashing against the glass at 35 mph, as fast as a male lion can run, we set up an experiment shooting padded steel into glass, again and again and again. The interior laminate cracked -- but it was still structurally sound."
Other than human error -- a door left unlocked -- the greatest source of danger for the Minnesota Zoo may be a sudden storm uprooting a tree, which pushes down a fence around one of the zoo's "shoot-to-kill" animals, those considered so dangerous that there are standing orders to gun them down if any ever gets loose. The caches of weapons are hidden around the premises for that purpose.
The zoo has had some practice with its new alarm: the self-designed system has had some hiccups, including a shakedown phase in which wireless dialers had to be replaced with hard-wired ones because they kept triggering false alarms.
"We had a false alarm at 3 a.m.," Fisher said, "and we had people here within four to five minutes. You know, there's a misconception that zoo animals are pets. They are far from pets. If you get within an enclosure, you are food to them, plain and simple. Even keepers are not their 'friends.' There is no bond. They wouldn't think twice about eating you."