There's a bunch of other stuff, too, but a $6 million grant will partially fund "an overhead passageway allowing the Zoo's big cats to travel and explore far from their usual habitats."

Big Cat Crossing is expected to open in the spring of 2014.

"This new trail system and other improvements will redefine what it means to be a 'world-class' zoo," Mayor Michael Nutter said in a statement. "As the Philadelphia Zoo's national reputation rises, it continues to anchor the historic West Fairmount Park neighborhood and serve the citizens of our city by providing a unique learning experience for youth and adults."

New initiatives at the zoo were profiled in the New York Times in June. 

Zoo president Vikram Dewan is calling the William Penn Foundation grant, announced Monday, "transformative." Foundation officials say the grant to the zoo fits into a broader investment strategy for West Fairmount Park.

Disclosure: The William Penn Foundation supports WHYY/NewsWorks.