The United States did not drop bunker-busting bombs on the nuclear facility in Isfahan, central Iran - despite assessments that the facility holds most of the country's enriched uranium - because it believed they would not be effective due to the facility's depth underground. This was reported by CNN, citing three sources present at a briefing by Chairman of the Joint Chiefs of Staff Dan Kane, who told this to US senators. The report noted that US assessments indicate that 60% of the enriched uranium - which Iran needs to produce a nuclear bomb - is stored in this facility.