I'm taking a course which is designed with the CISSP certification in mind. Though the class is categorized as software engineering, we talked a lot about physical security and, in particular, floods, fires, earthquakes and cars running into things. How is this security? For example we were told that data centres are safest in the middle of a building because if the roof was leaking the top wouldn't be safe, and water tends to go down so the lower floors would be the first to flood.
Is this really a security issue? For example, if the roof leaks the engineer would be at fault, not the security analyst. You wouldn't hire a security analyst to make sure the roof is solid.
UPDATE: Also, things like having bollards around a building to protect pedestrians from cars, how's this security? No one's really explained this yet. I was told the companies most valuable assets are their employees, but with this line of reasoning, what isn't security?
If the availability is so encompassing, what isn't part of security?

All other answers are fine. I'm going to offer you a classic security perspective.


Starting a fire/flood is a textbook scenario for physical penetration/exfiltration. People under stress are less likely to challenge strangers.
A fire can be used to destroy forensic evidence, in particular when there's insider involvement.
An earthquake or, indeed, any natural disaster (like bush fires) is a potential complication for security because law and order break down and looting rears its ugly head.
Perimeter security against SVBIEDs is a necessary consideration in certain countries and threat environments. If a suicide car bomber can drive close to the walls of your data center, it is your failure as a security consultant. Hence bollards, flowerbeds, and concrete barriers.
Security is a holistic discipline. Every specialist cares about bits and pieces of the enterprise, and by necessity of life loses sight of the whole. There should be at least one person out there who thinks in terms of adversary's behavior and not his/her own pigeonhole. Which, incidentally, is a security consultant's job description. 

