[vincent_byrne]

Hi Kentucky
I recommend that this issue be looked at from a financial and security perspective. There are no other reasons to put a data centre underground unless you already have ownership of a bunker or mine.
The cost to build below ground from greenfield is very high. They include excavation and perimeter piling cost and rock extraction where necessary. In must locations you will be building below the water table so the room will act like a ship and want to float so additional ballast will be required to keep it from popping up. In addition it will require special treatment to ensure it is water tight.
Security
The big Pro for a below ground data centre is the raised level of security which can be offered but air conditioning to me is always the weakest link. The air conditioning plant will still need access to outside air so this high level of security is easily negated by accessing the external plant or blocking air from getting to the external plant and its not like we can open any doors or windows.

Cooling
A below ground data centre will undoubtably be more insulated from solar gain than its above ground counterpart. In practice the effect of solar gain does not penetrate more that about 300mm below ground over the 12 hours from midday to midnight depending on soil make up. So that below that level the temperature remains constant and will settle at a level which is dependant on the time of year and the local climate. In locations where the soil temperatures are below the room temperature then we will have a cooling effect being delivered to the room along the side walls of the room which are exposed to outside soil which although it should not be ignored it will not play a hugh factor in providing cooling to the room. This is turn is compared to the heat gain of a well insulated conrete above ground structure which will be exposed to high daytime summer air temperatures. Let us consider the number of hours above say 21deg throughout the year to be say 1500.
Even if your cooling load differential for this time period was to average at as high as 100kw for a 1000sqm datacentre, with an external plant COP of 3.3 this would equate to $6,400 per year at avg US electricity prices.($0.128/kwh)
I have no exact prices for the additional cost of construction below ground for this size building but excavation and disposal alone would cost in the region of $200,000 requiring a 30 year return and that is the thin edge of the additional construction cost. I would expect it to be three to four times that figure by the time you are all in.

This is just a quick exercise to show that the figures just dont stack up when it comes to building underground. Nevermind the very high risks of flooding.

I know of only one such below ground data centre being built in the world at the moment and that is a military facility in the middle east and in making their decision to go underground there was a whole lot more going on there than just good design.
(For those who can come up with more precise construction figures please provide so that we can firm up on payback periods)
Vincen