Natanz is Iran's primary nuclear enrichment facility, with buildings both above and below ground. Facilities are built underground for two reasons: to prevent satellite surveillance and to protect from air or missile strikes. Although construction progress cannot be directly observed from space, we can watch the growth of spoil piles, which are piles of soil and rock excavated from the mountain. From the size and rate of growth of these spoil piles, inferences can be made about the scale and quantity of underground facilities being built.
Change in spoil pile extent over three years.
You can read more about Natanz and the analysis done by CNS in this AP News article.
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