Google posted a video tour of one of its data centers today, highlighting some of the steps it takes to keep data secure and maintain reliability.

Google has been notoriously tight-lipped about its data centers, and doesn't even disclose exactly how many it operates.

So why the sudden openness? There are several likely reasons:

• Yesterday's Amazon Web Services outage is creating new questions about the viability of cloud computing -- including services like Google Apps

• Also yesterday, environmental activists Greenpeace released a scathing report on data center efficiency and use of dirty energy sources like coal (PDF here). The group admitted it didn't have all the information it needed to make the report perfectly accurate, but blamed data center operators for not sharing enough information. The group had some particularly harsh words for Google, giving it an F on transparency.

• Last week, Microsoft called into question whether Google Apps was secure enough for government use. Google's slow response made it look like it had something to hide.

• Earlier this month, Facebook made Google look silly by open-sourcing the details of its own highly efficient data center design. Facebook's move is also meant to undercut Google's big data center advantage by letting the rest of the industry work together to improve efficiency.

Google isn't exactly going the Facebook route, but today's video is a big step given how secretive the company has been so far.