Three reasons, temperature stability, propensity to have rock vs sediment, and in the bay area, somewhat better earthquake resistance. When I was doing ops at B

Three reasons, temperature stability, propensity to have rock vs sediment, and in the bay area, somewhat better earthquake resistance. When I was doing ops at Blekko we had a dark fiber run between our office and the data center to get our own "high speed" connection and part of that was a geo-technical survey. Of much of the SF Bay Area peninsula. The geologist suggested if we wanted to be subterranean and avoid both infrastructure and superfund sites, that we'd have to go down 500'. He expressed doubt that anyone would do that given the expense. And there are still some parts of the Santa Clara aquifer you can hit if you don't aim well.