aquifer

Gene J. Puskar / AP

Pittsburghers have long been fascinated with the mysterious, underground “fourth river.” As much as they gush about the three visible rivers, they’re often eager to tell you about the secret waterway beneath the Golden Triangle.

Keith Srakocic / AP Photo

  From the highest elevations of Mt. Washington — about 1,200 feet — you can take in the whole sweep of the river valleys, each about twice as wide as the rivers running through them, said Charlie Jones, lecturer in the University of Pittsburgh’s department of geology and planetary science.  

“So then the question is why is the valley wider than the river? And the answer is: the ice ages,” he said.