Good point to have underground data center. In real world there are so many parameters which decided how and where to built datacenter.
Geographical location and local weather decides design of building. Having underground datacenter can reduce the cost of cooling if datacenter is hot geography but can increase the risk of flood if datacenter is in flood zone.
Normally construction cost for underground facility is more than elevate floors.
Some more points are highlighted in following checklist.
http://www.datacentertalk.com/2011/1...ity-checklist/