One of the biggest draws of the National Zoo is the O Line, also known as the Orangutan Transport System. It's two steel cables that stretch nearly 500 feet across two separate orangutan habitats.

The bridge may be a hit with visitors, but it's actually a great design solution. When faced with two half-sized enclosures with a wide distance between them, zoo planners solved the problem by creating the O Line, which allows the apes to move easily between both locations — and earn some cheers from the onlookers below.