Silicon Valley's booms set fire to the local real estate market and new construction followed. But after a year or so of recession, south bay is littered with ghost buildings and empty office complexes.

At 20.5%, vacancy rates are the highest they've been in the Valley since 2003. Vacancy in research and developments spaces is at 18.9%, according to CB Richard Ellis. In Sunnyvale, more than half of the available offices are empty.

Combined, a total of 41.7 million square feet of R&D and office space lay bare in Silicon Valley. That's twice the size of Monaco.

See some of the most massive unused office parks in Silicon Valley →