A suspected Iranian nuclear site, Pickaxe Mountain, has reportedly witnessed a surge in construction activity this year, according to analysts.

The site, situated near the Natanz nuclear complex and about 140 miles south of Tehran, has been under the watchful eyes of intelligence agencies as a likely location for Iran to secure its future nuclear program.

The Center for Strategic and International Studies (CSIS) think tank carried out an analysis of six years’ worth of satellite imagery. The analysis, reported by CNN on Wednesday, showed a “clear surge in construction activity” at the site this year. The underground facility is “likely” intended to serve as a safe haven for Iran’s nuclear program, according to the analysts at CSIS.

The report also underscored the challenges presented by the granite mountain in which the facility is embedded. The CSIS team reported that even the Pentagon’s “heaviest non-nuclear” bombs would find it a difficult target.

The analysts further indicated that the construction at Pickaxe Mountain has shifted from active excavation to likely internal construction and ongoing exterior reinforcement.