Question: How deep do they dig for a Security Clearance?
Answer: It depends on whether you are being processed for a Confidential, Secret, or Top SecretClearance. It also depends on whether or not you are going to be cleared for SCI (Special Compartmented Information) or not.
For an overview of how the Department of Defense (DOD) processes security clearances, see our article, Security Clearance Secrets article.

