Vacant skyscrapers, empty trains: can San Francisco once again reinvent itself?

The economic challenges of the city's downtown have sparked speculation of a so-called doom loop'. But some say its emptiness may be part of the solution

The operator of the gondola that services Salesforce Park, an oasis among the skyscrapers in downtown San Francisco, will tell you all about how the 5-acre (2-hectare) rooftop space you're about to enter contains 1,600 plants, 600 trees and more than a dozen ecosystems.

From this far up the fortress walls, the city looks like a futuristic utopia, with office workers milling about in the sun and free yoga on Fridays.