The lower part on the left side used to be part of the visitor area, but was made part of the Chimpanzee housing to connect the previously existing elevated exhibits to the new outdoor exhibits that was opened a few years ago.
@DelacoursLangur For the gorillas they had to come up with yet another solution. There they had to construct a tunnel underneath the visitor walkway to connect the gorillas' large "sunroom" indoor exhibit to the outdoor exhibit.