sarahnaomi writes: For years, San Francisco has had a robust fiber optic infrastructure laying dormant underneath its streets. Google announced Wednesday that it's going to start lighting some of those cables up. Welcome to the future of broadband in major cities. Most people don't know that many cities throughout the United States are already wired with "dark fiber": infrastructure that, for a variety of reasons, is never used to provide gigabit connections to actual residents. This fiber is often laid by companies you rarely hear about, like Zayo and Level 3, which lay fiber infrastructure in hopes the city, a provider like Google, or a corporate customer (like an office building) will eventually make use of it.