Members of the defense establishment consistently place HDBTs as the top priority for long-range precision strike. Finding ways to hold HDBTs at risk is a major defense priority because, as the NPR notes, 'at present the United States lacks adequate means to deal with these strategic facilities'. According to recent intelligence estimates, more than 70 countries now use underground facilities for military purposes, with over 10,000 facilities worldwide. Approximately 1,400 of these facilities house strategic sites, including WMD facilities, ballistic missile basing and leadership or top echelon command and control. HDBTs are classified into three categories -- 'cut and cover', simple tunnels and deeply buried facilities -- that can range in depth from 20m to 1km below ground.

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Miller, Eric A. "Global Strike Capabilities: The Ballistic Missile Option." Astropolitics. Vol. 2 (2004): 1-31. [ 5 quotes ] [ page 6 ]