Animal Kingdom is built in a bowl.
During Traditions (aka orientation), we were told that the park is sunken into the ground to create a climate suitable for all the animals in the park. This is also why the temperature inside the park is much warmer than outside. However, I haven't been able to find confirmation elsewhere on the Internet, so I'm only going by what we were told. I do know, though, that the animals are why this park generally closes first of the four at the Walt Disney World Resort.