This is one of the most interesting tours I have ever done and would highly recommend it. That said, finding the start point was difficult - it is not in the underground station but opposite the park, but once we located it, it was well worth it. No photos allowed in the underground, but a great experience all the same and the guide was very knowledgeable and engaging.
The zoo is impressive, allow a good 4-5 hours to explore everything. The new polar bear exhibit is still under construction however it appears it will be the best exhibit once opened. We rode the Ferris Wheel at dusk - wonderful views of the city and the surrounding fairground. Take your time too enjoy the Ferris Wheel museum located just after the ticket counter.