Well, I was getting ready to write a review of the Philadelphia Zoo, but it looks like I'll have to push that back until I see how this newest feature works out.
Every once in a while, technology, showmanship, and genius come together in a zoo setting to produce a new way to experience animals. Underwater viewing has changed the way that we exhibit a range of animals, from hippos and crocodiles to penguins and polar bears (and, in at least one German zoo I've seen pictures of, elephants). Reversed lighting has revolutionized the exhibition of nocturnal animals. A big trend which a lot of zoos - Louisville, Point Defiance, Denver - have started is the rotating exhibit, moving animals between enclosures. It provides the animals with more space and more variety, while also promoting activity and exploration.
Philadelphia, it seems, has added a new element to the equation. They aren't the first zoo to do something like this (at the very least there is National Zoo with its orangutans), but they might be the first to plan an entire zoo along these lines.
There are a lot of questions to be answered still. How much will the animals use them? How will other animals react to the sight of a tiger or jaguar strolling around the zoo? What if animals sit down in the middle of the tunnels and refuse to move? What will they be like to maintain and clean? And, of course, what happens when Mr. Lion decides to take a potty break with a crowd of school children directly beneath him?
All will be revealed in time, I'm sure.